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
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Genos / Tyros5 / HK Lucas Nano 600 / FTB Maxx 40a / EV ZX1A / Rock'n'Roller cart / Hauptwerk virtual pipe organ / misc other audio & music toys