1. Keep two wrenches for the ball mount. Not just one. You sometimes need one for the balls and one for the nuts.
If you often switch between 1 7/8 and 2" setup, you might consider just getting two receiver pieces instead of always trying to change those things with a wrench. That is what I have going on.
2. Trailer light wiring kit and a few connectors and tools you might need in a pinch. I keep a wire crimper, wire stripper, and cutter. A pair of needle nose pliers. Also, make sure all of this stuff is stainless steel if you can.
3. Make sure you have a jack that will properly lift the loaded trailer if you need to change a tire. I think the stock jack will probably suffice, but you should verify when you pick it up. I once had to jack up my enclosed cargo trailer in the rain on the side of the MA turnpike and we had to use a Fender guitar amp as a jack stand and use the trailer's A-pillar jack to raise from the other end...in the rain!
4. Rags/towels/degreaser/cleaner. The grease gets everywhere sometimes. You don't want to get back in that nice rig with dirty hands.
5. Before you put that trailer in the water, I would suggest COMPLETELY coating your trailer leaf springs with grease. I mean like about a tube or two PER side. Cake it on there. If you don't, those things will only last a few years if you live in salt water areas. Also, don't buy the trailer if it is not galvanized, unless you are doing fresh water only.
6. Find out about your trailer bearings!!!!! Make sure they are newer and in good shape before your trip back north. This is the number one reason I see boats parked on the side of the road around here. If they are the sealed type, then have a local shop repack them. If they are the other kind, just make sure the grease them up real good, and that they are not old. I replace the ones on my boat trailer about once a year simply as a safety measure. If a bearing goes out, your whole wheel will come off the trailer and the axle will grind on the road potentially causing a nasty situation. Happens all the time around here (coastal SC).
That's all I can think of right now.
Good luck!