If by usual things you are talking about Air and Space, Lincoln Memorial ext? How old is your daughter, is she interested in visiting any of the schools in the area?
You should have no problem seeing a few sights during your visit. Your best transportation method really depends on where you want to go. If you want to stay around the Smithson/Capitol area taking the bus/metro option (see details at the bottom of my post) should be fine. You can walk from one museum to another with out a problem. I would also recommend contacting your senator or congressperson in advance to arrange for a capitol tour, if you go with the House side odds are very high you will get to meet your rep, I can't say the same thing about Senate unless you are from a small state. Senators from bigger states tend to have set times to meet with constituents and odds are the one day you will be there wont be one of those times.
If you want to visit the monuments and see more of the city I would recommend renting a car. That said I would only go with this option if it is a weekend. Make sure you have a GPS!!! On a weekday there is just too much traffic, and driving will ruin your day.
If cost is less of a factor you can consider taking taxis everywhere, it has been a while since I have taken a taxi to IAD but I except they would cost about $60.
I would highly recommend not taking the Super Shuttle. They will drive around dropping a lot of people off and will eat into valuable tour time.
cheapest: 5A Metro bus, it is 3.10 cash each way per person and takes you to the Royslan Metro station. From there you can take the subway just about anywhere in the city. This bus can get very packed at peak times.
Taxi: Expensive probably about $60 each way, but they can drop you off right in front of your first attraction saving time.
rent a car: If you are good with directions and will be traveling on a weekend you may want to consider renting a car. Weekend rates are low and you have a lot of freedom. I wouldn't recommend this on a weekday because of traffic.