A collectable machine (or one that has been cared for and is in good condition), with all original accessories, ought not to be case modded in my opinion.
Some alternative options:
1. Put A/UX on it and play with an interesting old unix -
http://aux-penelope.com
2. Run it as a webserver -
http://www.ld8.org/servers/servers_68040.html
3. Run a webserver on Apache under A/UX and see how high you can take it by tweaking css
4. Tun it as an AppleTalk router with Apple Internet Router software (you've got zones!)
5. Run it as a print server to a localtalk printer like a LaserWriter 4/600 with AppleShare software, and as an AppleTalk router as well, giving you access on your network to the 4/600 over ethernet
6. The usual word processing/ gaming etc. machine, but used largely for experimenting with older software
7. Nubus card tester (if that turns your crank)
8. Learn Tex, using an early version of OzTex
9. BSD box
10. FileMaker 4 machine for cataloguing all your Mac gear
11. Anything else you can think of that you can't do on something like a G3
12. All of the above, one after the other.
The real way to make it worth keeping around is to give it a "specialist" job to do that also makes you learn how to do something new.
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