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.