Havent tried every app but If I had to think of some:
- an offline foss calendar that can import ics files (simple calendar is the only option soon fossify)
- an app that can display Checksums for files
- a dark privacy overlay for phones that can't use privacy screen protectors, like newer gen pixels
Edit: Second an offline fitness tracker where a person can manually input info. Been using an ods file for a couple of years but its a pain to use collabora on android
Have you tried Quillpad?