First of all the essentials: a couple of different types of lube, gloves, soft paper towels, nail clipper and file. Perhaps some condoms, depending on personal preference. I'd bring some massage oil because massages are really nice even if they aren't anal-related.
The other stuff depends on what you want to do and how much "risk of discovery" you're willing to take, especially when doing air travel.
Some toys are good to have, I'd take a small plug, larger plug, and a couple of dildos/probes of various sizes.
If you want to do pegging throw in a harness.
If you want to do fisting throw in some really thick lube.
If you want to do protected rimming then non-microwave plastic wrap makes a good barrier.
If you want to clean out first bring an enema syringe.
You might also want to take a couple of your own towels. Every once and a while things get a bit messy, and I wouldn't want to push that off on the housekeeping staff.
Lastly, plenty of other sex fun stuff. There will be days when your butt needs a break or you want to do something else. Also bring some non-sex stuff to build the other parts of your relationship.