Most of our road trip emergency supplies live there (more water in the summer, more warm layers in the winter).
I'll add on that this is where I keep the 'car' supplies. Shop towels and window cleaner to clean the windshield while supercharging, portable air compressor and emergency light, in winter our shovel and brush (this is really important because open any cabin access including trunk and you'll get snow in the car), our charging cables, the jug of wiper fluid, dust pan and brush, interior wipes. Plus my hiking poles and boots. Anything I might need to access without having to shift our luggage to get access to the sub-trunk compartments.
On road trips not in the dead of winter, I'll put a case of soft drinks in there too but this is Canada so temps aren't as crazy high as the US south. I've never noticed the cans are overly warm when I pull them out but I wouldn't drink from them without ice. I'm not concerned they'd spoil from the heat.
I agree, do not put perishables in the frunk in summer. It is simply not worth the risk.
Don't forget, the trunk's sub floor cover can be removed to give you more height if the dogs are in crates. There are two compartments back there, so you can remove one or both of the covers. And if you are putting the right side rear seat down, one tip I learned on TMC is to put a tennis ball over the exposed 'bolt' of the upright left seatback. Although, probably that is a bad idea with a dog (!), perhaps cut a piece of pool noodle to fit it.
(If you haven't already, buy covers for the rear seat vents and then carefully pack under the front seats for more storage of small things, although on my last trip I was able to put a case of 18 drinks mostly under there because I didn't need the vent to be uncovered.)
