I was going to put one of my smaller tool boxes in there, probably 12x6x8 or so. Once I posted this I went out and looked under my hood and realized there's nowhere for my box without moving a lot of stuff around. I can't put it under or behind the seat either, I have a standard cab.
Another thought I had come up with was mounting it somewhere up on the frame under the truck so it's out of sight, but that would be a pain in the ass to get to. I might have to get really creative to figure this out.
As far as what tools I'm taking: couple of screwdrivers and a few kinds of pliers, small socket set, hopefully some wrenches. Basically anything I would need to do any minor work I might need to do.