I have an amazing amount of stuff under the back seat: extra oil, a spout, and tire "goop" (Stan's NoTubes) in a plastic tool box; jumper cables; tool kit; chocks; hand cleaner wipes; 2-step ladder; tie-down anchors (the Claw); jack, and jackpoint. In the baggage area, I keep the survival kit; the tie-down straps in a stuff sack; the cowl plugs, pitot and AOA probe covers, fuel cap covers, aileron locks, and windshield cover in another stuff sack. Behind the tail cone cover, I have a bicycle pump clamped to the shelf.
Under the passenger seat, in a tray that can be slid out for access, I have a Kleenex box, sick sacks, and a box of Travel Johns. On the back of the passenger seat is the O2 set and accessories. Under the pilot seat and extending into the footwell of the backseat, I have the towbar. On the back of the pilot seat, I have a Nelson pouchpack, which holds various manuals, gloves, fuel tester, fuel stick, pocket knife, PLB, miscellaneous charging cables, etc. On the front of the pilot seat, I have a water bottle holder and fire extinguisher.
I've found that it's easier to put most of the things that are under the backseat and passenger seat under there without putting them in containers (other than the bottles of oil and Notubes, in case they leak), because containers themselves take up too much room.
Cary