There's the Roland GO:KEYS with regular-sized keys, or the much smaller Yamaha reface CP with mini keys. Both have on-board speakers and can be battery-powered.

There's the Roland GO:KEYS with regular-sized keys, or the much smaller Yamaha reface CP with mini keys. Both have on-board speakers and can be battery-powered. The reface would be the perfect size to play in an airplane seat, for example. I use the Yamaha reface YC as I'm into organs, and I really like it.