The only customer installable parts on a iBook are the RAM and the Airport card. You can max out the iBook's RAM with a 1GB SO-DIMM (abt $300 last I looked). G3 iBooks can get the older Airport card, and the G4 iBooks take the Airport Extreme card. Too boring for you?
Well, if you're brave (and patient) enough and willing to do away with whatever remaining warranty you have, you could upgrade the hard drive to either a 80 gig 5400 RPM or a 60 gig 7200 RPM drive. Also if you look hard enough you can find bare Superdrives that will fit either the G3 iBooks (tray-loading) or the G4 iBooks (slot-loading).
Not up to doing this stuff yourself? Check out
http://www.powerbook1.com who will upgrade the HD and optical drive for you if you mail the iBook in with plenty of money.
That's about it for upgrade options. Customizing. . . well, thats another story alltogether!
Hope I was of help.