You can take an elevator to the top of the Old Post Office Building to see a wonderful view. This might be a great spot for a "pitstop." On the main floor is a food court. Do not confuse this building with the Postal Museum next to Union Station.

I immediately thought of a bike tour of the monuments for your group. It saves the feet and is more fun than walking. Even a private one may be comparable to the bus tour and you would get more out of it while going at your own pace. I also suggest the Spy Museum and the Native American Museum. The admission prices are a little high, but you can see some stuff that young boys would like. For a better price, the Natural History area of the Smithsonian is good. Steer toward the animals, etc. Remember, DC was built on a swamp and it is hot, humid and hot at the end of July. Build in some times where you will get to go inside. We love Mt. Vernon too and the boat trip from DC to there is fun for kids. Have fun!

<<<the Native American Museum. The admission prices are a little high>>>

Are you referring to the Museum of the American Indian? If so, it's free, as is the American Museum of Natural History (and all the other Smithsonian museums).

I presume Mutti meant the Spy Museum, which does charge a significant admission.