There's nothing interesting in Rosslyn itself, however it's probably the closest subway stop to Georgetown. You can easily walk across the Key Bridge, which is a really nice walk when the weather is good. There's lots to see in Georgetown, especially restaurants, and also a nice waterfront area.
You can also walk from Rosslyn to the Mt. Vernon trail, and to the nearby Roosevelt Island where there's some nice hiking trails and a big monument to Teddy Roosevelt in the middle where people like to sit and relax. There's fountains there, running at some times during the year. Then you can continue walking on the Mt. Vernon trail to Gravelly Point next to DCA airport and watch airplanes take off. If you want to keep walking, you can go to Old Town Alexandria, or even farther to Mount Vernon, but it's a lot better to use a bicycle for these treks.
