Zacambú is one of the closest reserves to Leticia, about 70km away by boat. Its lodge is on Laguna Zacambú, just off Río Yavarí, on the Peruvian side of the river. Accommodations are simple, with small rooms and shared bathrooms, and a total capacity of about 30 guests. Plan on at least US$100 per person per night including food and transportation.
Zacambú sits on a flooded forest that is a habitat for many species of butterflies, but unfortunately it's also a popular hangout for mosquitoes. Its proximity to Peruvian communities also means it's not the best spot for wildlife. You are better off at its newer lodge on the Amazon, the Tupana Arü Ü, 60km from Leticia on the Colombian side of the border; you'll need a Colombian visa if you're visiting the latter.
Most excursions are made by motorboat or canoe, for obvious reasons – the bird-watching is particularly good.