(I remembered this yesterday while writing a comment about something else, but LeechBlock stopped me before I was able to write it here.)

The black keys on a piano keyboard form a pentatonic scale; that is to say, so long as you have an anywhere-near-decent sense of rhythm, nearly anything you can improvise using those keys alone will sound good. Non-musicians will be pretty unlikely to notice what you're doing, if they aren't very close to you.