I have found that looking at the thread of what did you do to your FRS today:
http://www.ft86club.com/forums/showthread.php?t=11036
And member journals:
http://www.ft86club.com/forums/forumdisplay.php?f=40
are a great source of ideas for what you may eventually want to do to your car.
There is a wide assortment of things and prices that one can do.
Personally, I have only gone with low cost mods: Tint, exhaust tips, Console plaque (to fill the divot in the console where the heated seat switches would be (if the FR-S had them)), LED lights for the dome, trunk, license plate, and reverse lights.
Just keep yours tasteful, otherwise, you might want to put your flamesuit on.