Which model do you have? I've got a 02 Forester which has the same dash as the classic WRX's, I've pulled out the plastic surrounds & painted them in a similar silver to the outside. Like so...
Change the interior lights to LED's?
Retrim door cards or seats in another colour?
Change colour of the dash & instrument lights?
Checkerplate supercheap floor mats?
Turbo timer?