Harmonica is also a great instrument for an adult to learn, but only if you have a place to go where nobody will hear you. It’s a great instrument for blues. But it takes a lot of work on technique to develop the basic licks that make playing fun. Bending notes is like whistling: some just never get it, others pick it up right away, but most have to fiddle with it a lot and slowly go from sucky to not terrible to not half bad to decent. Folks can give you tips on what to try, but for the most part, you have to try random stuff until it starts to work, and move from there.
It’s not particularly good for accompaniment. Also, it’s a bit of an investment, if you’re going to play with others. One harp is cheap, but to play with others, you need one for each key you’ll play in; a starter kit of at least 4 is minimal, for playing “cross harp” in A, C, G, and E. (For which you need D, F, C, and A harps.)
There are lots of good tutorials. After a couple years, you can menace the local blues jams. 
No recommendations for tuba? Darn.
(Just bought one yesterday, and it was as absurdist an event as you might imagine. I don’t think I’ve ever seen more than two tubas at once, and a room full of them - shelves, floor, racks - was as crazy as a Marx Brothers movie.)
Maybe not what the OP was after, though.
I hated walking to and from school wearing that Sousaphone. Lucky for me, I only had to do it once a week.
Let me add to that. Any experienced acoustic guitarist would be good in helping pick out a uke, even if he’s never touched one himself. He’d be able to check out the action, the tuners, and how the strings attach to the bridge/tailpiece. I said acoustic because there’s a lot more that’s adjustable on an electric than there is on an acoustic. Some stuff on an acoustic you either have to pay a lot to get adjusted, or you have to learn to live with.
To be honest, I was kind of buying that video until that came up on the screen (ya know, it’s Moog, they could have figured something out). After that, I checked when it was posted, and it made a lot more sense. 
Of all the instruments mentioned so far, I’d have to add to the chorus saying the probably uke fits the bill the best, provided you can at least use your hands well enough to push down it’s very light strings. Unlike any of the horns, the harmonica or the theremin, it’s an instrument that you can actually accompany yourself with in non-sparse arrangements. A uke’s strings don’t really take any force at all, if you can drive a car across town comfortably, I don’t see why you can’t play a uke. Also, as mentioned above, they’re cheap. You can get a decent ukulele for less than $50, and you kind of have to be shopping the rarefied makes to spend more than $100.
OTOH, you really can get a guitar that has loose strings, provided you’re willing to compromise on looking like a rock star. In fact, a kid’s guitar of acceptable quality can get you pretty far if you’re looking to accompany yourself. I’ve played a toy guitar by that mfr. I’m not sure if it was the Lightning McQueen signature edition (which can be cured with some creativity), but it had the same tuners. It played with about the same string resistance as a ukulele. That kind of makes that video even more impressive, unless he learned to play on that guitar.
If you decide later (or who knows, maybe sooner) that you’re willing to spend a bit more than that, and you must have rock guitar, Fender makes a mini strat. I’ve played my nephew’s. It doesn’t require much more force than a ukulele (but it does require more), and it sounds damn good. I kept asking to play it. I kind of want to get one just because it’s so damn easy to play.
The guitar and the trumpet will get you laid, a lot. The tuba and the oboe might do the opposite. Choose wisely.