I have 2 carseats installed in my backseat, so it isn't easy to flip the seats down and use the behind the seat storage, yet. (will be once the kids age a bit).
I have some small plastic totes that I keep a trailer hitch, tie down gear, recovery gear, and a few tools in. I can fit 3 of the totes under there pretty easily, and they are easy to slide in and out as needed.
During hunting season I can fit most of my hunting gear under there and the plastic totes ride in the floor board since the kids feet don't even touch the totes yet.