Some fun stuff I've tried:
1) Learn how to patch your kernel. Try the Con Kolivas patch.
2) Try out Dropline Gnome
2b) If you like Dropline Gnome, try downloading their dropline-build-system and learning how to make packages with it. That's pretty fun, if you like tweaking stuff to run right. Read the mailing lists, since the CVS version is not the most recent version.
3) Try seeing what packages you can "get away with" removing from your system. Gauranteed to break something eventually. Then you get to try and fix it.
4) Learn bash scripting and make some scripts for cron to run. You can do rsyncs, come up with an update notification system, clean up your harddrive, backup your system.
5) Try out win4lin and wine. Maybe there are other things out there to enable widows compatibilty. Check it all out and see if you can get something working.
6) Try setting up Slackware on an old box. That can be enough of a challenge, but if you get it done, then you can play with a network instead of just a single computer. Then, you can try out stuff like NIS. If you need an old computer, check out
http://retrobox.com . The cheapest computer there I see today is $12!!! If you have money to burn, I see a Pentium II 266MHz 256 MB 3.2 GB with CD for $30!!!
7) Try compiling mplayer with all the codecs so you can watch all kinds of movies.
8) Try out a different window manager. Try getting it tweaked just how you like it.
9) Try re-installing your whole system and take extensive notes. After you're all done, configure a web server, make your notes into a web page, and share your experience with the world. People will comment on it, and you can use there comments to make even further improvements to your system.
THose are just a few ideas.