Even tho our youngest is 14, we still use the back seat too (just not for baby seats or strollers).
It's also been found that you can fit:
1) 28 cinder blocks neatly stacked - Tom
2) Basketball goal (out of the box) - Tom
3) 18 cartons of Girl Scout Cookies - Eric
4) 800 lbs of canned food - Eric
5) 48" Plasma TV - Tom
On a serious note: don't ever put anything in there where you can't close the rear door. Even if you secure it with rope or bungie cords, the hinges can't take the load while driving.