The Costanera Sur Ecological Reserve owes its creation to rubble from a highway construction project that was dumped into the river: As nature took over the landfill, new life emerged along the shoreline. Look out over the estuary—that spot on the horizon (on clear days, at least) is Uruguay. Glance back toward the city and admire the urban-rustic juxtaposition of cattails framing sleek steel towers. You could spend all day walking the trails here, so give yourself at least an hour; rent a bike at the main entrance to get around faster. There are free guided tours in Spanish on weekends, plus monthly bird-watching excursions and full-moon nighttime walks.