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