I have a ReMarkable tablet and it's pretty nice. It's large, yes, but you can get a keyboard and then it's a word processor and Linux terminal. Use emacs or ssh into a bigger machine. Plus it's just regular linux and you get root shell out of the box.

There are smaller devices in a similar vein, I hear good things about Kobo.

E-ink tablets are about the best thing I can think of here. Second best option is probably just an old Android device without a sim card.