Umbrella : An umbrella will come in handy, especially if your backpack is not waterproof. There is nothing worse than doing laundry only to have your entire bag get soaked through the next day in a storm.
Rain Cover: One thing that I did not bring was a rain cover for my backpack and I paid dearly. Everyone else in the group had the cover which kept everything dry when hiking from country to country. Most notably waiting on line for customs in Panama it down poured with nowhere to hide... entire bag soaked and I was not pleased.
Padlocks : Two padlocks should be purchased. One can be used for your hostel locker and the second for your day pack or main bag if it cannot fit in the locker. I used cable padlocks since they could easily go through my backpack zippers as well.
Earplugs : Hostels can be loud at night from people filtering in and out of the rooms and from the bar which could be right downstairs.
Water Bottle : Possibly one of the best items I brought on my trip. You cannot drink the water in Central America, so refill this whenever you can to save money each day! Also go with an insulated bottle, it will keep your water cooler for much longer.
Swiss Army Knife : Another fantastic purchase. The knife should have a bottle opener and scissors, two items that are needed often and under rated.
Tea Bags : Random I know, Placing tea bags in your backpack will get rid of the smell by keeping moisture out.
Duct Tape : I chose not to bring tape, but I am also not traveling on as small of a budget as others. I have seen people do amazing things with duct tape, such as fix their shoes and backpack. Even seen others create a water cover for their backpack with just tape and garbage bags.
Money Belt : I found these to be useless. Just keep your belongings in your front pocket and leave your passport locked at the hostel. Problem solved.