One of the most beautiful rivers in Central America is the Rio Dulce that separates Belize and Guatemala. The river is slow and deep as it comes down from the highlands and flattens out before reaching the sea at Livingston, the ramshackle port town on the Caribbean. The boat trip up the river is very pretty as it goes through the steep gorge, and in the morning and evening nearly every day birds, especially pelicans and tucans, migrate up and down the river. Several Mayan families live along the river, and the waterlogged villages, many on stilts over the river, welcome visitors. They often sell gift items, such as tiny hand-carved canoes, jewelry and fabrics.
It isn't far up the river to Lake Izabal with a resort on Catamaran Island, and just beyond that is a Spanish fortress, Castello San Felipe, gone to ruin.
How to get there? If you are packing a kayak--a lot of people going to Belize do--you can easily paddle up with an overnight stop in a village, and there are occasional trading boats that will take on passengers.