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.