180 tstat and Cold Air Intake like everyone else is saying
They are the primary ones that you should do before anything else and they are just about the cheapest too. For my CAI, all I did was buy a K&N High-Flow Filter that was shaped like my stock filter. It cost about $40.00 at AutoZone. The K&N Filter is cleanable and reusable so it saves you money over buying stock disposable filters. After cleaning the filter you have to treat it with K&N Filter Oil. The initial filter comes preoiled. I believe an extra can of K&N Filter Oil costs $10.00, but it lasts for a few cleanings.
My 180 Tstat I purchased is a Moto-Rad Fail-Safe Thermostat. That means when it goes bad, it will lock open so the water will still keep flowing. It cost me, $10.99 at AutoZone. When you replace the Tstat you need to replace the gasket. I believe, it will cost $2.00. I also had to install an additional paper gasket when I replaced my Tstat because I couldn't get a good seal when I put the Tstat Housing back on. Water was leaking slightly. I bought a generic sized paper gasket and had to cut it to fit properly. This cost another $1.00. That'* my story.