As many others have said, get a 3DS. I’d recommend either a new 3DS (non xl is my favorite but, well, small) or 2DS (not new, the new is a fragile bitch).

I also own lots of anbernic devices I’d recommend but not for that use. That would then require a quite powerful device but in the end lack the polish and use of a native 3DS.

You can also do… Things with any 2/3ds nowadays. Just in case you didn’t know.