Nickelmenu adds a fifth item in the bottom bar of Nickel (your kobo's stock UI), that opens a customizable menu from which you can launch custom apps and scripts.

Koreader is a full alternate UI for your e-reader. On top of being open source, I personally find it superior to Nickel for a few reasons (non-exhaustive list) :

• Its PDF reader :
  • It's a lot faster than the stock PDF reader, especially for large documents on which the stock reader becomes almost unusable
  • Has the ability to auto-crop PDFs with large margins
• It lets me organise and browse documents in a regular folder hierarchy
• It renders markdown files (which is handy if you write a lot of them)
• It supports wireless transfers using Calibre
• It has a Syncthing plugin (which let me sync my "notes" folder full of markdown files with all my other devices)
• It lets you run a SSH server on your e-reader

NickelMenu is generally used to launch Koreader (and other apps) from the stock UI. KFMon is an alternate way to launch third-party software by openning "fake e-books" from Nickel, but I did not try it. You should check out this thread on MobileRead by KOReader's author and maintainer for instructions on how to install both at once.

Other cool apps I'm using :

• I'm working on a fork of InkVT (a terminal emulator) to make it compatible with physical keyboards (I have an Elipsa 2E, which supports bluetooth). It still has a few quirks and do not want to share it until I've fixed them, but it gives me something that is somewhat usable for SSH-ing into anything, or run vim/nvim locally to write some notes or blog posts.
• rmkit (bundles a few apps along with an app-switcher) : It was initially made for Remakable e-readers. While it works on Kobo I found remux (the app-switcher) to be a bit unstable and it has a tendency to fail in a way that uses 100% CPU and drains the battery until it is restarted. I'm trying to fix these issues in a fork, but so far I've only managed to create new issues. What is nice about it is that it is designed to allow switching between apps without restarting them. I use it to mainly to switch between InkVT (to write) and KOReader (to read reference documents for whatever I'm writing)

By combining both + Koreader, I have