Retrothing has a paen to the Cambridge Z88, a clever little device made in 1987 that ran for 20 hours on four AA batteries. I honestly don’t know of a single device I own that can run that long. Besides a word processor you also had a version of BASIC built-in and a nice little LCD screen with big keyboard. Sure the 640×64 resolution is a little rough, but what more do you want?
If you think about it, everything else, including the iPad, sucks in comparison.
You can still buy them here and did you know that Douglas Adams wrote Last Chance to See on one?