Someone on here has a mod that feeds air more directly into the airbox, and there's a timing advance trick, too. The gains aren't huge, but it's something. I've also heard about guys replacing the V90 cams with the older 960 cams (higher HP, but lower bottom-end torque).

And if you want to get really stupid, there's this:
http://www.turbobricks.com/feature.php?content=winter_06

Though i don't think the last one qualifies as "easy."

The exhaust system on these cars is pretty good already, but I think it could stand for higher performance mufflers, if that's your thing.