Someone with more recent experience may want to chime in, but just four or five years ago I used to spend a lot of time in D.C. and cabs were really cheap! I remember paying $10 for four people going three or four miles. It was $1 or $1.50 for as many additional passengers as would fit.
The price was better than Metro and you got door to door service!
That was the old Zone System. Sometimes it worked in your favor (you were traveling within the same zone), other times you got fucked (you were traveling a short distance but charged more because you crossed into another zone, or you got a shady driver who cut across the corner of a third zone to jack up the charge).
There’s a panda cam website that provides a live feed of the bears scratching themselves and staring into space. You could check that out beforehand to decide whether they’re worth seeing live. Note that there will almost always be a long wait to enter the panda house during the summer.
There is an outdoor area which overlooks the panda pen. Not an option if the pandas don’t come out, of course.
We just got back from a MD/DC/VA trip (kids age 7 and 10). The DC part involved a lot of walking in hot and humid weather. We stayed at the Holiday Inn about 1 block south of the Air and Space Museum, which worked out well for us.
The Mall involves a LOT of walking. One day we walked from the hotel down to the Lincoln Memorial (about 1.75 miles). By the time we got there, everyone was hot and tired and my 10-year-old wasn’t feeling well and we had to take a taxi back to the hotel.
We stopped by the Bureau of Engraving and Printing to buy bags of shredded money. I would have liked to stay for the (free) tour, but the kids were getting restless. The Air and Space was crowded, but still a must-see IMHO. We also hit the Natural History Museum (the 7-year-old REALLY wanted to see the Hope Diamond), the WWII Memorial, the American Indian Museum (mostly just to eat at the cafeteria) and a bit of the Botanical Garden. We only had 2 days in DC, so that was about all we could manage, with all the walking in the heat/humidity. Kids had a great time though.
Agree with everyone about the Metro. We only took the Metro into and out of downtown so we just bought the paper tickets.
Another mode of transportation around the Mall is the pedicabs. We didn’t have a chance to take one, but our friends did, and they said it was fairly cheap.
I believe the Udvar-Hazy is free to enter but $15 to park. We thought about going there but ended up at the Manassas Civil War Battlefield in VA instead.
One more local chiming in on the Air and Space Annex as being better than the one on the mall – assuming you have a car. They have an SR-71 Blackbird there … what more do you need to know?
I will add one thing that nobody has said: unless you’re dead-set on Italian and/or unadventurous, DC is a really great city for exotic ethnic food. Not every town has a Nepalese restaurant.
Comments in suggestions made above:
Eamon’s is my favorite restaurant in Old Town. It’s Irish-style fish and chips.
The Mitsitam Cafe at the Natl Museum of the American Indian is the sole decent place to eat on the National Mall itself.
Restrooms in Metro stations—every station actually has one hidden away in the “backstage” area of the station where they have storage, mechanical rooms, employee lockers, etc.
it used to be that they didn’t let the public use the restrooms, but now if you ask they have to let you in.
But these restrooms are frequently out of order, so don’t count on them. I suspect “out of order” usually means a homeless person has made a mess that hasn’t been cleaned up yet.
This isn’t really a suggestion for the OP, but I thought that I’d toss it out there for other readers.
I’ve always loved Union Station, the architecture in the main hall is beautiful, and that cafe and bar on the level above is a cool place to go have a drink and relax.
I preferred the Air and Space Annex as an adult but the kids may differ. My five year old got bored on the tour relatively quickly. The main museum has interactive displays and other stuff that kids might like. The annex has a lot of planes but none of them are open. Additionally, the annex is in Dulles near the airport, so it isn’t close to anything, this means that going there would make it an almost all day affair, and would require the rental of a car. Absent a lot of time, I would skip this to see the must sees which are all on the mall fairly close by.
I like Geranio in Old Town Alexandria for Italian. I think that there are some pretty nice restaurants at National Harbor as well, but I haven’t spent much time there.
My favorite Italian place is Pines of Italy on Columbia Pike in Arlington. You’d have to take a bus or taxi to get there. It’s not upscale by any means but it has an interesting atmosphere.
I’ll second what others say that if you’re in the area, it’s almost a waste to go for Italian.
The area specializes in Ethiopian, Vietnamese, Korean, Salvadoran, and all other kinds of ethnic cuisine.
Hey, in fact, I’ll recommend a couple of places on Barracks Row—which is at the Eastern Market metro station.
Banana Cafe (Puerto Rican and Cuban) — http://www.bananacafedc.com/
Café Belga (Belgian) — http://www.belgacafe.com/
Good point.
Most unique place in town, IMO, if you want to give the wife an experience: http://www.yelp.com/biz/thai-x-ing-washington
Reservations very much required.
Another good-to-know Metro factoid is don’t eat on it. They aren’t arresting 12-year-old girls for eating french fries like they used to, but save your candy bars till you’re out of the station.
More’s the pity. I was always glad when people got arrested for eating on the Metro, it’s kind of a shithole now.
What do you mean? I don’t see people eating fries on the Metro.
I see people eating and drinking on the metro all the time, more importantly, I see their garbage Disgusting things People Do on Metro – Vol. 12 – Spit Sunflower Seeds - PoPville
So I take it breastfeeding is right out? 
I’ve noticed that more on certain lines than others.
Good to know - my son has a small bladder and was barely able to hold it until we made it out of the Metro station and found a nice secluded patch of trees near the station…
The National Building Museum (pay museum) has a giant maze exhibit. It’s pretty fun but for a family of 5 it could end up costing a pretty penny. The rest of the museum is pretty bare. There’s an exhibit on houses/homes, and an exhibit on building toys (essentially a toy room) and not much else. Just a thought to consider.