If you're into cycling, there's also a paved bike trail that runs from Rossyln all the way to Mt. Vernon. You can rent a bike in Georgetown, cross over the Key Bridge (walk your bike over that) and pedal down the bike trail to Mt. Vernon (with an optional ice cream stop along the way in Old Town Alexandria). I've done it before and it's about 18.5 miles from Rosslyn, one-way. It's mostly flat with only a few small, rolling hills. The part of the trail from Alexandria to Mt. Vernon is mostly shaded and is a pleasant ride when the rest of the area is baking in the summer sun. The trail also hugs the Potomac River all the way down, so it's very scenic.

I wouldn't suggest it if you haven't ridden 30 miles or more before, but if you're comfortable with @35 mile rides, it's very do-able with lots of places to rest along the way. There are also a couple of public bathrooms and porta-lets at various places along the trail. There are bike racks at the end of the trail at Mt. Vernon, so ask the bike rental place for a lock.

Another option if you come to DC a lot is to join the Zip car program. It's a program where you rent a car by the hour instead of by the day. There's a one-time $25 application fee and a $25 annual fee, so it's $50 upfront, then you pay by the hour ($8.75 in DC, but it includes gas and insurance). It's not worth it if you're only going to need a car for one or two days, but may be worth it if you come to DC or one of the other ZIP car cities (NYC, San Francisco, Boston, Toronto, Minneapolis, and Chapel Hill) frequently and need to go to areas not well served by public transit. They're having a special right now where they'll give new members a free $25 towards their initial hourly rental fees.

They park the cars near Metro stops and they're easy to get to. You just pick them up and go. When it's time to return the car, you park it in one of their prereserved spots around the city. They tend to use hybrid cars, so you're also polluting less, if that's a concern.

In addition to ZIP car, there's another, similiar, service in DC, but I can't remember the name right now. I'll post it if I can find it. As I said, if you're only going to use this service once or twice, it probably doesn't make sense financially. However, it may be a good option if you visit frequently and find yourself wanting a car for a couple of hours each time.