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.

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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