A Red Hook sludge boat: The Department of Environmental Protection has three boats in New York harbor whose sole purpose is to transport sludge, and this is your chance to tour the largest of the fleet. Just don't fall into the sludge, because whatever superhero you turn into thereafter will have really gross superpowers.
Solid Objectives - Idenburg Liu studio: Our neighbors down here in Dumbo, who designed Pole Dance at MoMA PS1 and won the recent Sukkah City competition, will have the doors to their studio at 68 Jay Street open all weekend from 12-4pm.
Newtown Creek Digester Eggs: Those two giant metal eggs (or breasts, depending on how you tend to visualize things) on the North Brooklyn border are the largest of the city's wastewater treatment plans. This is also conveniently, ironically close to the Newtown Creek "nature" walk starting point.
Brooklyn Lyceum: The massive, beautiful, barely salvaged from disrepair public bathhouse on Fourth Avenue will be all yours to explore from 11am to 6pm all Open House weekend, from the football field-sized basement to the ground floor performance space, and innumerable other areas we've always wondered about.
Pier Glass Studio: Behind-the-scenes tours of the Red Hook glass-blowing studio have already filled up, but you can still visit their beautiful facility and exhibition space, not to mention the Civil War-era warehouse in which it's located, between 10am and 5pm all weekend.