In general, wear what you should have worn with trousers in the same situation, only everything should, with a tartan kilt, be monochrome.
On your head
A cap, a sun hat or nothing.
Unless you are Scottish
and the situation or event calls for it, a bonnet or other Scottish headwear will most likely make a caricature out of you.
On your upper body
A
t-shirt, polo shirt, oxford shirt, in fact, any shirt you should otherwise have worn with trousers or shorts.
Shirt tucked-in or untucked?
Tucked-in is the general answer, and on which purists insist. And with Oxford shirts, I agree. However, men today are increasingly wearing t-shirts and polo shirts untucked when wearing jeans and shorts; why should it be different when wearing a kilt for casual wear? In general, I prefer tucked-in shirts.

Untucked T-shirt over kilt.
Depending on the temperature, a
sweater over your shirt is what to wear and always untucked, of course. A nice
hoodie is also a possibility.
As
outer wear on cold days a
short wind breaker (bomber jacket type) or
anorak will be perfect and look nice with your kilt.
Never wear a coat, covering your kilt and, like already mentioned, never one of your normal jackets which should be too long and look totally wrong.

Short windbreaker. No sporran. Caledonia is a universal or fashion tartan. No clan connotation, therefore. Green kilt hose matches the green in the tartan.
On your legs and feet
After Brexit, inexpensive items like kilt socks have become absurdly expensive for EU citizens. The reason is that the declaration fee might exceed the price of the socks. You can either buy more at a time to share the fee or look for alternatives.
Over-knees instead of kilt hose
If your shoe size is 39-43 (6.5-10 US; 6-9.5 UK), you may be able to find some thick over-the-knees socks, either in local women's stores or online. In off-white or charcoal, they are genderless, and they can be faster and easier to get. In general, prices are attractive, and no taxes or fees need to come on top.

Many over-knees are making the perfect kilt socks. These are from H & M. They are by far more durable than any original kilt socks I have ever worn. Actually, for probably ten years, my preferred kilt socks have been such over-knees socks!
Unfortunately, H & M doesn’t have them in all markets or every year.
5-yard kilt in 13 oz. pure new wool, Farquharson tartan, worn with thick 'kilt socks' from H & M.
Must I wear kilt socks when temperatures are high?
According to the purists, YES, but the 'Kilt Police' shall probably accept them
scrunched down.
In that case, you can wear suitable, cheaper knee-high socks. When scrunched down, nobody can tell the difference, and the knee-highs shall, due to less material, feel more comfortable.
Also, on a hot day I think that
short socks or
“invisible” socks or
no socks are making a lot more sense than thick woollen kilt hose - provided the kilt is worn as casual wear and not to a wedding where a dress code must be followed.
Could I wear knee-high socks, which are thinner than kilt hose?
No, not if rules are to be followed. But if you think your calves must be covered and temperatures are high, and you are probably the only man wearing a kilt, which is almost always the case, I don't see why you should not wear knee-highs.
But optically,
thin, non-transparent over-knees socks, which can be folded down under your knees like kilt hose, might be a better choice than knee socks. Then you must visit a women's store, however, or order online.

Clockwise, starting upper left; 1) ordinary kilt hose, 2) short socks, 3) invisible socks, 4) knee-high socks.
Can I wear tights or leggings on a cold day?
The YouGov research, referred to earlier, indicated that some kilt wearers will wear tights and leggings under their kilts. On kilt forums, where it has been discussed, some are OK with it; others (most) are against. But after all, you can see men wearing them under shorts, so why not? They should be non-transparent, of course. And you might consider wearing kilt hose over your tights.
When HRM King George IV in 1842 visited Scotland (it was the first visit of a British monarch to Scotland in 183 years) he, at the grand ball, was wearing pink tights, in a newspaper described as "buff coloured trowsers like flesh to imitate his Royal knees under a rather shortish (meaning too short) kilt".
Tights on cold days
In my opinion, kilt socks are looking better, but if you feel more comfortable wearing tights or leggings when it's cold outside, do it.
Also, from my own experience, I know that on bitterly cold days, most people find tights or leggings the ONLY way to go.
For that reason, and though I seldom feel cold, I sometimes choose to cover my knees with tights or leggings.
Tights as a fashion statement
Given that we are discussing casual wear, and the kilt is not a national symbol to you, I see no
rational reason that should prevent you from wearing opaque tights or leggings under your kilt, should you feel like it. Too femme? I think they go, but it depends on your society as well as your overall appearance.
I have seen pictures of men in kilts with transparent skin-coloured tights complemented by ordinary kilt socks.
In
my eyes, they didn't look too feminine. You hardly see them, unless they are shiny. However, even subtle differences can make you cross the line.
Footwear
Wear shoes fitting the purpose. For casual occasions, sneakers are often a suitable choice. Heavy-duty shoes and walking boots are also popular. Or loafers and dress shoes. Simply, wear shoes which you would have worn at similar occasions and in trousers.
If I let go of the sporran?
A shoulder bag is the obvious choice. Men already wear them, so no big deal about that and suitable ones should be easy to find. For example
Eastpack makes some nice shoulder bags. You can have them in about the same size as a sporran, or you can have them larger to accommodate an iPad Mini, a regular iPad or a laptop. And in many colours. They are all unisex.
Often, you’ll have pockets enough in windbreakers, etc.
Holyrood kilt worn with sporran.
Holyrood kilt with a shoulder bag.
In crowded environments with pickpockets around, the most secure place for your belongings might be the sporran for obvious reasons.
A shoulder bag should better be worn as a crossbody; and be closed, of course.
By the way, the kilt itself, being unusual and thus an eye-catcher to some extent, might, compared to neutral jeans and shorts, provide some added security against pickpockets, these types wanting to operate as unnoticed as possible.
Colour coordinate
Think colour scheme when wearing a kilt. Colours of other clothing items should always be in harmony with one of the colours in your tartan or match them.
Under your kilt
What you wear – or don’t wear - under your kilt (or trousers) is a matter of personal preference. But the idea of wearing the kilt “like a true Scotsman” reguarily might appeal to many. Probably every kilt wearer has tried 'going without'
at least once, this being part of the game, so to speak.
However, before you ditch your underpants, keep in mind: Wool kilts must be dry cleaned, and even if some PV kilts are machine washable, they may still require a lot of ironing.
Considering that almost half of all kilt wearers (according to earlier-mentioned YouGov research) will not wear regular underwear under their kilt, there should exist great market potential for a solution that can overcome the hygiene problem while still ‘letting them feel the breeze’.
The solution is called a
kilt liner. But even if it makes a lot of sense, obviously, nobody is making or selling them anymore.
The company, which has recently stopped selling them, had a very old-fashioned website, which was a good reason why it couldn't continue.
Another reason might be that kilt wearers have found other solutions regarding the hygiene question. Or they wear their kilt so seldom that the hygiene factor is not an issue.
Another reason might be that kilt wearers have found other solutions regarding the hygiene question. Or they wear their kilt so seldom that the hygiene factor is not an issue.
A kilt liner is, or was, simply a thin skirt or a half slip. Instead, a so-called
skater skirt will do the job. And this might be what many kilt wearers have realised and practice.
Skater skirts are typically made of a jersey material. This one is from H & M.
A skater skirt under a kilt
Such a skirt is, contrary to what the ‘authorised’ kilt liner was, wide, really wide, and accordingly much more like a kilt, in fact. The elastic waist as well as the jersey fabric are what you are familiar with from your underwear or sports clothes. So, nothing feminine per se about that. When worn under a kilt, a skater skirt is just open, roomy underwear, which shall protect your precious kilt efficiently against you.
The skirt shown under the kilt above is grey, but unless the kilt is grey, a black skirt is preferred, because it better 'disappears' into the shadow under your kilt.
But how about comfort, you may ask? Try it out. I guess you'll find the
“semi-commando” solution coming very close to the ‘real thing’.
However, if you think tradition must be followed, forget this advice and do what you must.
To mention: On windy days or when you have to sit a lot, there certainly are better, more straightforward solutions than going full- or semi-commando. And even then, the kilt is a most comfortable garment.
Under your kilt? You know. Your wife will find out. For others, let it be left to their imagination.