Z1 intake tubes+K&N filter or a cold air intake. This would be the cheapest nonexhaust power mod. I'd expect 10-15 hp out of that.
If you drive hard, I think DOT4 brake fluid is a good one. I used to always be scared my fluid would boil coming down from the dragon. Switched to ATE Super Blue, which is pretty cheap and much better than OEM fluid.
I also really liked putting in Swift springs. Reduced body roll quite a bit and lowered the car perfectly for me. Swaybars could be good too. Next to an exhaust, suspension changed the feel of my car the most.
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