Yes, marijuana is (somewhat) legal in D.C., but you still can’t smoke it on Federal grounds. Plus those are family-friendly, historic places – so do everyone a favor and just don’t.
When D.C. was designed, nobody planned for millions of visitors a year. This particularly applies to the sidewalks. Do everyone a favor and walk with your group single-file in tight places. Realistically, you should never walk more than two people across, and even then, stick to the right. Locals are always in a hurry, and it would go a long way for tourists not to block the sidewalks. If you want to stop to take a picture, step out of the way.
When using the metro, heed the “doors closing” announcement. It is not a warning, it is not a suggestion. The doors are closing, and they will close on you. Do not try to hold them for people, do not try to rush through. This will delay the trains and make you incredibly unpopular on your car. Plus, your group could get split up if you time it wrong. Just wait for the next train.
Another metro-specific tip: on escalators, walk on the left, stand on the right. The worst thing you can do is stand side-by-side with your friend while commuters are trying to barrel down the escalator and make it into the office on time. You’ll have plenty of time to chat while waiting for the train.
Traveling is already expensive, but you need to account for the high cost of living in D.C. when planning your trip. A $8 glass of wine is going to run you about $12 here. Metro trips add up, Ubers surge, and that’s before you get those replicas of the Constitution for everyone in your family. Plan ahead so you can have a stress-free trip.