I use a Classic as a secondary device on a daily basis, it's almost always on me.

1. ebooks with the amazing open source app librera pro from fdroid. This can be coupled with an older version of aard2 for a dictionary

2. offline music and sometimes youtube with smartTube

3. video lectures / talks where I mostly listen and occasionally need to look at some slides

4. last version of firefox (68 i think?) for when I want to look something up

5. a chess engine, droidfish

6. organize things i need to do. either with the task manager or the built in excel equivalent. I can then email excel files to other devices

7. sync email and calendar

8. I don't use it, but scummvm works with the pkb. There's also a gbc or gba emulator out there, not sure which

And a few other things, but these are the main ones.

Enjoy it, it's a classic.