Some great ideas
So just to reinforce those already posted:
A new desktop case that could house the old motherboard. New, more ergonomic keyboard. Different colours (black!), designs etc.
Memory board that could fit old 4 or 8 meg SIMMs (everyone has a couple lying around). PCMCIA aware.
Flashrom kickstart replacement (just like the one Toni described). There's already software to put together custom ROMs.
Wireless network card (clockport) and updated drivers that could handle WPA2 encryption.
PS2 joypad adapter.