I'm off to Thailand with my son shortly, and I needed a day bag that is big enough to carry things like a water bottle, sunscreen and a waterproof poncho as well as all the other essentials, but not too big.
I used the Sew Sweetness Widget bag pattern, but I modified it so that it can be worn on the waist if required but can also be worn crossbody style.
I used waterproof fabric and waterproof zips (its monsoon season!) and there are lots of pockets in there - the one on the back is for passports - the one on the front has a key fob holder, card slots and another zipped pocket. The main compartment has a mesh zipped pocket as well as a standard zip pocket.