Swing Through the Treetops - Saturday, May 7 - Travel 100 feet into the air for a look over the treetops; then take boat rides or canoe trips, hike on nature trails, explore science labs open with demonstrations, jump on a hay ride, seine, crab, catch oysters; plus enjoy food and more at the Smithsonian Environmental Center Open House. Park at Central Middle School, Mayo Rd. and Rt. 214 in Edgewater; then hop the free shuttle. 9am-5pm @ Smithsonian Environmental Research Center, Edgewater. free: 443-482-2218; www.serc.si.edu.