Take a friend who brings along something else to ride. Give him/her your list (add tow rope to it) and tell them that's what you're planning to bring. Then don't bring any thing and mooch whatever you need off them.
Take a small cooler bunge corded to your frame. Put some wire, some bolts of common sizes, vice grip, screwdriver, allen wrench, a small hammer, a spare chain link or two, and a cresent wrench in a plastic bag in the cooler with your food and drink. Another bag with a phone and camera if you want to record the event. Put a 5 or 10 lb sledge hammer, basic socket/wrench set, fix a flat/battery powered compressor, and jackstands in the car. If it worse than what that list will fix, you'll probably need to go home to fix it.
Personally, I leave most of that sort of stuff in the car and try not to get too far away from it in case I need to walk. I'm usually more worried about the trailer breaking down on the highway, so I grease it up before I leave, double check the lights and safety chains, and make sure the spare has air in it.