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.

There are foldable or modular 88-key keyboards:

https://carryonplaying.com/product/carry-on-folding-piano/

https://pianodevoyage.com/

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.

Expensive, but the Teenage Engineering OP-1 is great if you can swing it.

Battery for days and days and completely self-contained. You can create entire songs on the go.

Though maybe more of a synth than portable piano — depends on what you’re looking for.

With the caveat that I don't own one, how about a microKORG? Apparently the most popular synthesizer ever.

Perhaps a roll-up piano? List:

* https://philadelphiapianoinstitute.com/roll-up-piano-keyboar...