I bought an old Yamaha 3-octave, small-keys sample player keyboard years ago off Ebay. Has 2 little built-in speakers, runs on batteries. I keep it by my computer - and I pull it out often to figure something out, play along with a song of interest, check things when I'm working on a song, etc. etc. It's been very handy. It "eats" batteries, so I use rechargeables.

I suppose any little, cheap/old keyboard would suffice for this kind of need ... maybe even a "toy" keyboard - anything you can grab easily/quickly and as long as it's A440 tuned and you can plunk out a few notes ...

-Jim