If you do your own work, take a good tool kit. If not, save the space for something else and just take the kit that came on the bike. Rain suits. Even though it is summer, if you get drowned going down the road in high country, it can be cold. Cell phone. Take 2, take a spare battery, take a charging system for the phone! I have forgotten those before. Any idea how much an I-phone charger is in Guatemala? (not on the bike, but you get the picture) Take a camera, with HUGE storage, and take lots of pictures. You are young now, and one day you will not be able to find the time for a trip like this. Get some of them TSA legal toiletries. You don't need a big 'ol bottle of anything (well, a couple might not hurt
Take a tire pressure gauge that works and you trust, and check you tires cold, every morning before you hit the road. A comm system would be great. since you have a passenger. If not, throw in the ear buds (take a spare set) and load up the MP3 with your favorite music. Makes the good scenery, and the bad scenery go by smooth. Take the time with your girl to work thru some simple hand signals if you don't have a comm rig, so you can communicate. I can almost guarantee she will want to stop more than you.
Hope this helps. I'm a bit jealous. Love to get my woman on the road for a run like that sometime.