Everyone loves Rock Creek Park and the dozens of other great green spaces in the District. We have waterfront spaces, forested spaces, and open fields all over the DC. But one District green space is accessible only by travelling through Virginia.
Theodore Roosevelt Island is in the middle of the Potomac River. Since it is located away from the shoreline, it is technically part of the District, not Virginia, whose land claims end at the waterfront. It's mostly composed of undisturbed wooded areas, except for: the memorial statue and grounds for Roosevelt, gravel trails around the island, and a portion of Roosevelt Bridge which carries I-66 and US Route 50 across the Potomac.
To access the island by foot or bike, depart from the Rosslyn Metro station by travelling north on N Moore St (towards 19th; you should be able to see the National Cathedral in the distance). Make a right onto 19th ST N and then a left onto N Lynn Street. Just before the Key Bridge, make a right onto Mount Vernon Trail. The entrance to the island is just after the pedestrian bridge over GW Parkway. All told, it's about a 15-20 minute walk from Metro the the entrance. FYI - you can bike to the island, but you'll have to lock up in the parking lot; no bikes allowed on the island itself. Enjoy!