Ranking enviro-friendliness of transport.

So, I've not got much time, but let's start this off anyway.
What's the most environmentally sound method of travel in DC?

Obviously holding top position is walking or cycling. Both are zero-emissions and you can go anywhere you need to if it isn't too far, or if you have the endurance for it.

Riding a bicycle is much faster than walking, and though the current condition of roads is far from perfect for cyclists, DC's working on improving the situation. The Washington Area Bicyclist Association announced not too long ago that DC was named by Bicycling Magazine as most improved bicycling city in America. There are many exciting improvements planned, and surely more as others pick up some human-powered wheels!
It's a good idea to ride the bike lanes when possible, even if it puts you a bit out of your way. Some roads are just not fun to ride - potholes, dense traffic, poor visibility... fortunately, the DC Department of Transportation puts out a map that charts which roads have been rated in terms of bikeability (yes, probably made that word up). It's a good resource, check it out.

However, there's the added benefit of being able to take in one's surroundings when you walk. You can't exactly take photos when you're riding, and stopping someplace for a drink means dismounting and finding a place to lock up your bike. But it's fun! And it's great exercise!
Yes, and there are others which you may prefer: skateboarding, rollerblading, street luging - is this legal in DC? even possible in flat downtown? I take pause here.

So, use your head and use your legs.