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Jump on a cargo boat and go to Guayaramerín, at the border with Brasil. This is unforgettable experience to see river Mamore and surroundings (the river is winding), and to experience daily life of cargo boat men. The trip may take 5-7 days in dry season, when there's less water in the river. With more water, the trip is faster, i.e. 3 days. Boats leave when full. For where and when to jump in, ask at Distrito Naval (naval neighborhood; 6 de Agosto corner Meliton Villavicencio) in Trinidad. Look for captainships (capitanias) for ports Almacen and Barador. They will direct you to a corresponding port, or maybe port Heralda. Go there, and deal the price directly. With information, guys in Beni tourist information office can help too, but then they'll direct you to "distrito naval".
Cathedral established by Jesuits, rebuilt over the years.
Ethnoarcheological museum, where you can see regional costumes (mostly on dolls), and find some information (photos, models) over the ancient hydro-civilization of Beni. It may smell like petrol inside the main building, because the floor is cleaned with a very common cleanser that it based on petroleum. You can also see some pretty gigantic lily pads here in the pond outside. You can jump in i.e. if you wait for a shared taxi to San Joaquin and have some spare time.
Beni Fish Museum
See a wide variety of living and preserved turtles, fish, and other species representative of the Amazon Basin, including a healed river dolphin. Guided tours available.
Historical museum of town and area, lots of woodwork (tables, furniture pieces, etc).
This is a plaza in honor of ranchers and ranching life, where you can feel free to climb atop a cow statue for that unique photo you always wanted. Across the way is the Plaza el Aviador where you can watch planes take off and land.