I've been using LinkSheet. You set it as your default browser to have more control over links you click in external apps. For example, you can manually choose which browser to open a link in each time or have it remember to open links from a domain in a specific browser. It also has other nifty features like built-in clearURLs, Libaredirect, and others. Settings are stored locally and can be imported/exported.
This is really useful when trying to compartamentalize web browser activity.
I also use Red Moon as an offline FOSS alternative to Twilight since I'm super sensitive to blue light. I use it so I can have a 3rd preset filter. During the day, dark mode but no filter. In the early evening, night light turns on. If I decide to use my phone while in bed in my dark room later that night, I use a combination of night light, extra dim, and red moon.
Red Moon is essentially abandoned by the developer, but it's a very simple app that still works well and probably doesn't require much maintanence anyway. It also doesn't require network access.
Not open source, at least I don't think, but Weawow as my weather app. Its on the Play Store. I use this instead of a FOSS app like Breezy Weather since Weawow is one of the only free android apps that still supports Dark Sky ever since Apple poached it to integrate it into Apple Weather. The app itself is lightweight, doesn't require any permissions besides Network to run properly, and doesn't require play services either. It also supports manually setting a location instead of requiring location access.
You can also run it from the browser instead of installing it.
https://weawow.com/