Parking can be expensive, and so can taxis, I haven't really traveled it before, but I think the Metro is pretty good

The Metro system is good. It has AC, it's clean-ish, and their underground stations are nice and cavernous, though it does seem rather dark compared to other subways. It does charge by the mile, however, so remember to consult the maps in the station for the fares. If you have to go far on the Metro, it can be a little pricey, but nothing like driving or taxis. Never drive in DC unless you know what you're doing. Traffic is terrible and drivers can be aggressive. Parking is a hassle to find and can get quite expensive. The roads can be confusing if you don't know how they planned it out, and there aren't always traffic signs where they should be. Pedestrian traffic doesn't always pay attention, so you have to constantly be on the lookout for people jaywalking. As for taxis, remember they have to deal with that sort of traffic every day, so they price accordingly. There are bike rentals if you feel up to it. As long as you remember that the traffic is insane and watch out accordingly, it can be an interesting way to get exercise and see the sights. There are also segway tours, if you want to be a part of a tour, and also bike tours. I used to live there, so I've never been on one of those myself, but they do look like fun.