There are a lot of cool add-ons and mods, but for absolute must-haves, I'd say my top three are:
- NickelMenu: The gateway to enabling other mods and add-ons.
- NickelSeries: If you sideload books but don't use Calibre to transfer over books, this is invaluable as it will properly populate the Series field in the UI. Useful even if you do use Calibre as you can hold off on that second reconnect if you're in a rush, or is useful if you download books via the official Dropbox functionality.
- Better Hyphenation: The stock hyphenation dictionary is sub-optimal. These dictionaries have more rules and saner default options.
And if you like to tinker with the UI or want to enable some hidden features, honourable mention to
kobopatch.
Edit: And if you're looking for a good,
free alternative or supplement to the built-in reading dictionaries,
these dictionaries based on
Wiktionary data come pre-made in Kobo format and are updated twice a month & generated nightly (save the dicthtml.zip files to the .kobo/custom-dict folder).