For your wheels:
http://www.renntech.org/forums/tutorials/article/32-home-curb-rash-repair/
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As for a quick and easy mod, desnorkle your car.
10min. Boxster "de-snorkel" Hack! :: Car & Lifestyle Forum
+5Whp Dyno confirmed. Did on my car and no regrets!
I have also heard of other forum members who have drilled holes in their muffler exhaust to great acclaim, but I have not personally done that.
http://986forum.com/forums/diy-project-guides/19770-diy-exhaust-mod-%96-gain-10-hp.html