Really, anything you do such as AI or Exhaust, even headers will provide nominal increases in power at best. For real performance gains, you need to take the mods to the internals and work your way out.

To answer your question though, there are some air, fule, and ignition management bolt on mods that will help things out a bit... i.e. Fuel Pressure Regulator, hi-flow fuel pumps & filters, injectors, fuel rails, NOS, programmable ECU/ECM's, ignition control modules, coils, distributors, etc. etc. etc. Furthermore, you got things like headers, cat back systems, AI, adjustable cam gears, cam shafts(a little more work then straight bolt on), etc. etc. etc.

None of this stuff is cheap though, the cheapest things like AI and the like help when combined with other key components but don't make any big performance gains when used alone.

IMO the best place to start tuning a N/A motor with simple bolt ons is by managing it's fule, ignition, and air. If you want to take it to the next level though, you really need to get inside the motor or take the easy way out and slap a turbo on (easy in terms of one mod providing a lot of power, not easy in terms of fine tuning and setting it up properly).