I would recommend checking out the circulator bus. Very inexpensive and easy to use.
http://www.dccirculator.com/
Personally, I HATED the night monuments tour. The trolley was very hot and packed to the gills, and you start out riding around the Capitol and such; takes quite awhile to get to the monuments. I actually bailed at the Lincoln Memorial and then just walked the rest of them and took a cab back to my hotel.
What I like doing when touring in DC is to use Capital Bikeshare. There are 4 bike stations near the monuments, so it's pretty easy to find a bike or to return one. The minimum age is 16. https://www.capitalbikeshare.com/
Thanks for the replies guys!! We do want to do the zoo as my mom as never seen the pandas. We probably won't spend too much time there though. On the night tour of the monuments, do you go by the White House? And is the Vietnam Memorial lit up at night? My daughter and I did a night tour a few years ago, but we did not really go by the White House and the Vietnam Memorial was dark. I walked the length of the wall, but could not read any of the names. The only light was from the Lincoln Memorial and a few lights from the park and surrounding area.
We just got back from DC and we used this company to do a night time tour of the main monuments. White House, Vietnam memorial, WW2 Memorial, Washington Monument, Lincoln Memorial, Korean Memorial. Vietnam Memorial is hard to see at night so he recommended we come back during the day but all the rest of them were gorgeous at night, especially the WW2 memorial. We had Dan King and he was awesome and very informative. He takes you to each area, talks for a few minutes about the history of the memorial or monument then lets you explore for 10-15 minutes and then you are off to the next one. It was a total of 2 hours. There were kids on our tour and he had them engaged and asking questions.
We stayed at National Harbor MD and took a Lyft into DC. Once in DC we walked everywhere. We also did the new African American museum and the Natural History museum but I didn't realize the dinosaur area is being redone and won't open until 2019. Again we just walked it.
The company also offers tours of Arlington and other areas. If I were to do those tours I would just Lyft into that area and walk the tour.
Which tour did you use? The link you provided lists several. Thanks for the info!!