I'm gonna be real with you: none of the instruments are easy in the way that you describe.

The easiest instrument for you to be able to play would honestly be the piano. it is the easiest instrument to play by design, and people don't even realize it. It really only gets hard when you try playing separate parts with both hands, which isn't a requirement.

You're literally hitting buttons, which we do elsewhere in life all the time. Heck, I'm doing it right now!

You can't just pick up an instrument and sound good. That's not how they work.

If you want to just be able to make music in a pub without having to work at it, use a phone. Otherwise, you'll need to put in a great deal of time and effort.

That being out of the way: guitar. It's an instrument with loads of resources on how to play, doesn't need to be accompanied by other instruments, and is very controllable when it comes to volume.

And you don't have to be amazing to play something fun.

The simplest wind instruments are things like your basic wood flutes, recorders, ocarinas... simplest stringed instruments are going to be 3 and 4 stringed stuff like cigar box guitars, tenor guitars, and ukuleles. But none of them are just pick up and play.

Realistically even the simplest instruments are going to require at least 50 to 100 hours of practice and instruction to reach an advanced beginner / early intermediate level, where you start to be able to intuitively do things that sound good. The great news is that good YouTube teachers can help you reach that point, if you're willing to put in the time and practice.

I learned to play guitar and tenor guitar in my 30s, and it's been extremely rewarding to just be able to unwind and create some music whenever the mood strikes. It was work... but enjoyable work, if that makes sense? I liked the practice and getting the results of my progression. I'd definitely recommend learning to play an instrument to anyone with the time, patience, and inclination.

Tambourine only demands a good rhythm.

A very basic "Learn Harmonica" book can get you far enough to improv in the middle of a band and not sound like trash.

I have a few ideas...

Ukulele is so simple it is often taught in grade school. And it has a lot of capability to grow into.

An autoharp can make great accompanyment to almost every popular song from the last 100 years.

Lap dulcimers are often set to one key - making simple songs a matter of sliding up and down the fretboard.

But my favorite is the stick dulcimer. The most famous is the Strumstick. Also sometimes sold as cigar box guitars or other odd names. If you want to just strum out the simple tunes, it's unbeatable. Watch a few Strumstick Bob videos. It's not bullshit. Just don't drop full price on a new name brand stick as it's easy to find beautiful instruments inappropriate for a raw beginner.

Hit me up if you have questions.

A lute would probably be easiest to learn the lute but I am biased I think