I've had my eyes on something like Akai MPK Mini, should fit in a backpack with a laptop (12.5" wide) but doesn't do its own synthesis so you're gonna need both

I've had my eyes on something like Akai MPK Mini, should fit in a backpack with a laptop (12.5" wide) but doesn't do its own synthesis so you're gonna need both devices (and I guess decent headphones as well), even if you just want to practice for a couple of minutes. Otherwise it looks great: 25 keys, 8 pads, pitch bend, USB-powered, all the standard assortment of features.