Not exactly *great* ideas, but:
You could dump a decent hard drive in it and run it as a NAS server. Check out the free diskette bootable NASLIte+
http://serverelements.com/ I use this app for a backup / mp3 jukebox storage.
Or if it has PCMCIA (two nic cards), make it a linux firewall / router.
I also used an old thinkpad as a Wardriving rig. Installed the Auditor Collection
http://new.remote-exploit.org/index.php/Auditor_main with an Orinoco gold card.
Any of the Live Linux bootable CD distros is fun to play with on old hardware. Knoppix, KnoppixMAME, Helix, etc)
If you can stick a wireless nic in it, maybe configure it as a wireless access point?
Or as mentioned earlier - stress testing it with firecrackers, ability to leap tall buildings, stop a locomotive?