Nothing! A good old fashion tune up.
Seafoam your car, change the plugs, check for vacuum leaks, check gaskets on your intake manifold , etc. Its a 99, its almost 11 years old, plastics and rubber dont last forever. The car has been through about 25 seasons of hot and cold, itll wear shit out faster than you think.
If you still want to put shit in your car, run Lucas Automotive gas treatment only.