2004-2005 Bonneville GXP rims, carbon fiber dash and door trim pieces, slotted and/or cross-drilled brake rotors, and I'm sure there'* more, but that'* what I can think of for stock parts upgrades.
What you can do to upgrade what you have is take your air box and gut it. PontiacDad did it to his 2000 SSEi and said it did wonders. others have done it as well and all seem to have benefited. Not quite as good as a CAI (Cold Air Intake), but it'* a good way to free up 2-3 hp, and better throttle response.