Seriously, there isn't a whole lot you can do, other than... STi seats. I guess if you look real hard you can find the OE carbon fiber trim kit for like $100. You can get STi pedals. A shift knob too... oh wait, nevermind.
Ummm... new RS floor mats are like $65 I think, they look good. Exterior is a little easier to deal with. Get yourself a nice V5 lip, paint your skirts, rear spats... some decent wheels, clear corners, etc.