Probably the most useful thing for many, is installing entware. It's a package manager that allow you to install many other programs on your router, most likely not found in your firmware.
I have my own project called tomatoware I use. I use it primarily for using asterisk, a telephony program I use for google voice.
You can hook a USB printer to it, which will allow all computers print to it through your router.
Some people plug in a USB audio jack, and have it play music or other things.
You can plug a hard drive in, and use it as a NAS. For a while I used mine for Usenet (or torrenting). It was also my media server, and I could stream videos to my tv from it.
Lots of fun stuff you can play with.