And also why? Some recommendations? Some app that you think that is not as famous as it should be? Etc etc
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
MuPDF viewer is a simple pdf/cbz viewer. One nice thing about it is how easy it is to open multiple documents at the same time. If you use your tablet for document review, this is a nice feature. The big downside is that the built-in file browser can’t access SD cards.
Ghost Commander is a dual-panel file manager that is useful if you move files around on your device fairly often. It supports SD cards, although write access can vary depending on Android version.
Termux is useful if you’re used to command-line tools. It supports bash, zsh, etc and the built-in package manager is good. I have successfully used ssh-related commands, vim, emacs, lynx, elinks, newsboat, cmus, mc, ncdu, and other programs from the desktop world on my android devices. Very useful, especially with Hacker’s Keyboard, which makes it easy to enter CTL, ALT and F1-F12 keystrokes on an Android Device.
Librera Reader is a very nice option for ebooks, comics and PDFs. It has read support for SD cards and works well with built-in text-to-speech if you like to have a robot voice read your books aloud to you while while you drive or work out. It reminds me a lot of Moon+ Reader, but it’s available on F-Droid.
Flym is a decent rss reader - I’ve been using it for a few years without incident.
VLC and Kodi are available through F-Droid if you’ve got a media collection on your device. Additionally, VLC can stream network-shared media if you’re on your home network and want to watch something that isn’t stored on your device. Also it can cast content if you have a chromecast on one of your TVs.
For gaming, you could download Dosbox or Scummvm (or add the Retroarch repository) if you have some old DOS games or you’ve downloaded some ScummVm-type adventure games from GOG and you want to play them on the go. The interface is a bit clunky but you can share the save files with your other devices using Syncthing.
Here’s a slightly more niche gaming recommendation - I’ve been enjoying OMW a lot lately. It’s one of several Android-focused ports of OpenMW that is available on F-Droid. With a bit of help from syncthing, you can play TES Morrowind on your PC and then pick up from the same spot later on your phone or tablet. The controls take a bit of getting used to, but it can be done! Who needs Skyrim when there’s an older, better alternative?