keep an eye on your coil packs, they are notorious for failure. if you get a bad misfire and check engine light suspect coils first. you might seafoam the engine. there was a thread around on a how to but i cannot find it anymore. basically warm up the car (by driving preferably) then shut car off, disconnect the vacuum line at the brake booster (on drivers side firewall, if you dont know) turn the car back on (it will idle a little bad) slowly pour about 1/3 to 1/2 the can into it. (dont allow it to die while doing this) then cut the car off and reconnect hose to booster, then let it sit 5-30 minutes and then go drive it (its going to smoke, and there maybe a lot, depending on how dirty the innards of the engine are) drive a little hard until smoke clears then drive normal after. then just enjoy the cleanliness you gave the engine.
for performance there is a little to do to help it. obviously air intake (either a drop in k &n filter, or whole kit) i have a drop in and it give a slightly noticeable bump, but i did it so i dont have to change it once a year. (save money)
then you could free up the exhaust some by buying mufflers performance exhaust kits ect.
iridium spark plugs come to mind ngk ix iridium part #5464 (or BKR5EIX-11) upgraded coils and wires.
clutch upgrade if you plan on racing (stock is kinda slow)
lower it, strut upgrades
many more just dont wanna make this so long