From Crystal City there are really two routes. I'd just use the Mount Vernon Trail and 14th St Bridge all thing being equal. It's cool to see all the things going on at the airport, and the planes come in low for landing over Gravelly Point. Also, there are good views of the DC "skyline" from across the Potomac as you head north along the trail from the airport to the 14th St Bridge, which empties out right next to the Jefferson. The alternate I used today is nice in its own way, especially for its view of the Pentagon and its surroundings, riding not too far from Arlington Cemetery and then the view of the Lincoln Memorial as you cross the Memorial Bridge, but the Old Jeff. Davis Hwy is a pathetic excuse for a paved street. I've mapped out both routes from near Crystal City metro to the Mall going both ways. Here are the links.

Along MVT: http://maps.google.com/maps?f=d&sour...&z=14&lci=bike

Alternate I took today: http://maps.google.com/maps/ms?ie=UT...&z=14&lci=bike

I'd offer to meet up with you as a fellow BFer and ride over together to show you the routes, but sadly my riding time is generally limited to my commuting miles. Hope the map helps, though. You might want to compare this map to the latest map of CaBi station locations to find the closest place to either river crossing to trade in your bike and/or hop onto a new one to avoid the extra cost of having a bike out for 30+ minutes.

If you want variety, I'd perhaps recommend doing a loop and going up one way and back the other?