Washington, DC is a heavily dog-owning town. The city is finally recognizing this by installing dog parks that legitimize the practices of letting your dogs run in a public fenced-in area, which was happening already.
Buy The Dog Walker's Handbook at http://www.lulu.com/content/paperback-book/the-dog-walkers-handbook/6144723 for all the tips and tricks for dog handling and care.
There is an official dog park at 11th and Florida Avenues NW that is large with lots of room for bigger dogs to run. Another fenced-in area right next to it is reserved for small dogs only. What a great idea. The little ones can run and play with each other without being intimidated by the size of the other dogs, which is when they will often turn aggressive.
There are bags to pick up poop and trash cans to throw it out.
The only problem I have is the owners of the regular dogs there aren't very engaged with their pets. I say this because they seem to have more interest in talking to each other and drinking coffee than watching their dogs. And the play can get a bit rough, too.
The ultimate message here is you need to watch your dog in a dog park no matter what. It doesn't matter that your dog plays with the same dogs every day. One day it could be different and there could be a fight. It's just what happens sometimes. They are animals, not people. And frankly, even people do that, too.