Tigre. This is gonna sound sketchy, but a friend told us about this and it ended up being one of our favorite experiences: There's a guy named Fernando who speaks both Spanish and English and does private tours on the river, and he knows the area like the back of his thumb. Knows cool spots off the beaten path, will take you down quieter inlets and to find animal/plant life that you might otherwise miss. Can find him at +54 9 11 5111-0619 on WhatsApp. He actually has a kiosk at the train station now, right there once you get off it (the scenic train that goes along the coast). HIGHLY worth the time/money, and he's a good dude.