Depending on budget:
Funky mousemat (with ergonomic wrist-rest)
Wireless anything...
Mac games
RAM (not exciting but always useful)
iPod sock (or to hell with it, iPod)
G5 2.5 with 8gb RAM with...
30" Cinema Display (beeeeeeaautiful)...
Hey... How about a donation to MacRumors Forums?!
(let them put up a 'tar, they'll love it)