Up to 3" blades are okay. No serrations, no fixed blades. Most of the museums have metal detectors now. You can't go in the White House any more. The Washington Monument has heavy security now, but the rest of the monuments you can still pretty much walk up to. Assume that you are being watched, all of the time.
DC is a police state, especially down around the Mall and other government buildings. There is surveillance EVERYWHERE, and there are plainclothes police, FBI and Secret Service agents EVERYWHERE. Don't believe me? Try walking up to the White House fence and taking a picture. The last time I was in DC with my in-laws, I tried to take a picture of them with the White House as a backdrop. 2 plainclothes officers (Secret Service?) swooped in from opposite directions and started giving us a hard time, telling us we weren't allowed to take pictures. I've been in that house! Several times as a kid. You used to be able to take tours. Now you can't even take pictures of it. It's sad. You would not believe the police presence in DC. There are very serious looking cops all over the place. They have the newest equipment. They are mostly young, alert and heavily muscled. They are all armed to the teeth, and in no mood to take anyone's crap. It's very different from how it used to be, and more than a little scary.
DC used to be a nice city, but now it sucks. Half the time traffic is being diverted for "security reasons" and the other half it's being intentionally disrupted by protestors. It's ridiculously expensive, and extremely difficult to get around in. The Metro system is falling apart, the roads are a nightmare. I used to live there, I grew up there, but I had to move away. It's just insane.