Waterproof bags
Used to separate and keep various items dry in your bag. Good for keeping valuables and electronics. Simple Ziploc style bags are good for this. However, much better are the plastic bags made by 
Aloksak.
These are very strong but lightweight, transparent storage bags. They come in four sizes: 7 x 4.5 inch, 6.75 x 6 inch, 9 x 6 inch and 12 x 12 inch.
They are made of a patented, super-stretch plastic, and very puncture-resistant. They close easily and are Waterproof to 200 feet.
Gifts
A final suggestion is to think about taking along some small items that you can present to people you encounter along the way. There is no faster way to endear yourself to the families you meet.
The colourful (economical, flat) press-on stickers you can generally buy near gift wrapping supplies are appropriate for many ages.
Another good idea is to carry a supply of the balloons used to make animals, hats, and the like. You’ve almost certainly seen this done, and perhaps never realized how easy a skill it is to learn (at least for simple items). T. Myers Magic will provide the instruction – including how to blow up the balloons without popping your eyeballs – and the supplies.
For adults in less industrialized areas, inexpensive (but not cheap, throwaway) ballpoint pens are a good choice. They also make good bargaining chips at markets.
High quality fish hooks will be very scarce in poor fishing communities, making them excellent gifts and trade goods (especially for fish!); make sure you get the flat variety.
Bandanas, not widely available outside North America, are another good choice.
Other adults may appreciate a souvenir (perhaps a small flag pin) of your home country, or maybe a interesting coin of your own currency.
