i have removed/replaced the rear seats and rear seat cushion quite a few times to run some fuel economy tests. The first time the bolts were a bit of a pain, try using a small piece of pipe or a piece of your jacks handle to slip over the ratchet and gain extra leverage. Wrap the end of the pipe in duct tape or some other item to prevent any scratches to your trunk paint. Once you have the seat back unbolted CAREFULLY slide it out the rear passenger door, i recommend folding the rear mounts up to reduce the overall size before attempting to slide out. Another side note is that the seat back is only anchored together on the bottom with a single pivot point, be careful not to let the 60 side of the 60/40 seat back go unsupported and potentially break at the pivot point.
as far as easy/cheap weight removal goes:
rear seat back was an estimated 35 pounds, and the rear cushion was about 5 pounds.
spare tire, jack + tool set, trunk carpet/styrofoam spare tire offset piece under the trunk carpet, as well as cargo cover are about 30-35 pounds combined if i recall correctly.
siphon out your windshield washer fluid, or remove the entire bottle if you aren't restricted by Scca class rules. (There's a surprising amount of fluid in it when full)
remove the glovebox and save about 2 pounds.
remove floor mats and save about 3 pounds.
if you have no passengers ever, remove passenger side head rest and save a pound.
as far as things that are free, that's probably about it as far as effort/reward are concerned. if you wanted to get down to the nitty gritty, you could remove the entire interior plastic/trim/carpet and save about 15 pounds combined for everything.
replacing the battery can save 30 pounds but cost about $200
replacing the rims will save up to about 20 pounds but cost ~$500+
removing the muffler and getting a pipe welded on in place will save about 11 pounds, but cost around $60
I'm sure the list is endless but off the top of my head here's a few examples.