I use a few apps from the SimpleMobileTools suite. They aren't full FOSS, they have basic and pro versions where the basic version is GPL3 and the proprietary extended features cost a few bucks. The basic FOSS tools are still decent, if barebones.
The suite includes:
- a calculator
- a phone dialer
- a music player
- a calendar
- a photo gallery (with basic editor)
- an audio recorder
- a flashlight
- a clock
- an app launcher
- an SMS messenger
- a camera
- a keyboard
- a note taker
- a file manager
- a contact book
- a simple painting canvas
I use the gallery and file manager the most. Though admittedly I threw a couple bucks their way for the proprietary extensions. It's not FOSS, but if it was going to be proprietary, I think it's one of the fairest deals in software these days. Better than another bloody subscription model, or holding the ad-free experience hostage behind a paywall.