We stayed at the Hoilday Inn Capitol last visit, 6 blocks from the capitol building and 3 blocks from the Smithsonian. The hotel has a tour desk that can arrange tours etc. see
www.hicapitoldc.com Sorry, but I can't give you a comparison of prices. It was a clean, convenient place that felt safe tho. There was also a store a few blocks away..seems like a Walgreens or something.
The Smithsonian (see
www.si.edu ) is a great deal of walking both inside and between the many buildings, but there is seating and even places to eat in/at some of the museums. As most building are handicapped accessable now, you might check with the hotel to see if wheelchairs are available for loan/rental. I know they are inside the museum, but I'm not sure if you can take them from one building to the next. I would recommend making a plan as to what buidings you want to visit prior, we waisted a bit of time without a map and you could spen weeks in the Smithsonion alone.
The Metro station is about a block away from the hotel. (I only remember escalators down to it, you might check if there is an elevator.) It is much cheaper than a taxi, and there are weekly passes that are cheaper than a per ride ticket. It can be a little confusing, so we learned to ask for "switching" info before we left the hotel.
www.vmata.comArlington is a must see. Tour group/busses are the best way, and you will get more of the facts from a tour guide and it is rolling with hills. Do see the changing of the guards rain or shine. The facts about the guards make you appreciate them even more.
www.arlingtoncemetary.mil www.tombguard.orgIf you want to visit the Capitol,(
www.visitthecapitol.gov )you might want to contact your congressman or representative's office to see if you can have a photo with them on the steps. They usually don't mind the PR if you schedule an appointment well ahead of time and they aren't currently in session.
Plan for a visit to "The Wall." If your family had a fallen Vietnam vet, you can search where there name appears on the wall at
www.thewall-usa.com . Take paper and wide led pencil to make a "rub" of the name(s)
For kids, Google some facts about DC and perhaps some of the places you intend to visit prior. It can provide entertainment on the trip as well as make things kind of "fit" when they see it in real life. There are even crosswords and games about the facts of DC. I don't know what ages (or if kids)
www.dc4kids.com is a good one.
I made a good investment of dollar tree rain capes and cheap umbrellas before we left (expensive there). Lighter than and not a hot as rain jackets. A zip lock bag(s) to shake em dry and fold for reuse in the day if needed. (I took extras as they do tear easily.) There are restrictions at the Capitol and some other places about large backpacks and what you can carry in (liquids etc). Google your plan to make sure.
Hope some of this is helpful and that you have a wonderful trip! It is one I would like to make many times again, there is so much to see and do.