Packing is one of my favorite parts of the trip-planning-process, partly because I'm a freak for organization, but also because it means you are getting very close to the trip! This is a great place to start, but I actually think there are a lot of things on there you don't need to bring. Don't bring items "in case" (like an umbrella)....you can buy that stuff during the trip if you do end up needing it. Try to keep your backpack as light as possible, around 20 lbs at the most. I'll definitely be taking all 6 of these things on my upcoming trip to Guatemala and Belize!
1. Headlamp — I brought this on a whim to my first backpacking trip to Europe and wouldn't travel without it again. From late night trips to the hostel bathroom, to the random cave adventure you didn't expect to go on, this is a very useful item!
2. Quick Dry Towel — These towels are super thin and light; they won't take up much space in your bag. They are great for water-related adventures, but also at hostels. Every now and then you need to rent towels and you can avoid the hassle and the extra expense with one of these.
3. Journal — You need a journal! Just trust me, you need a journal.
4. Nylon String — I use this mostly for laundry (in use in Peru here), and since it's virtually weightless and very small, it's no problem to add it to your bag. But I have no doubt the string will also prove valuable for more than just laundry.
5. Carabiners — I mostly used these to attach random things to my purse, like my water bottle or scarf, but these end up coming in handy for a million things you'd never expect!
6. Suction cups — I use the netted bag from my quick dry towel for my toiletries (shampoo and such). Throw a suction cup or two in the bag as well and use it in hostel shower to hang the toiletry bag from so you don't have to set it on the floor.