I travel quite a lot to DC and lived there for 10 years before moving to NH, and concur with the other posters and would like to add:
1. TourMobile is super. DD and I did it last April and we're going to do it again in June.
2. Parking in DC is costly during the week, they love to get the business travelers. Also, some hotels have specials on weekends that include parking. Before you pick your hotel, you could also ask if there is a nearby self-park garage which would be much less $$. You will not want to take your car out until you are leaving for good!
3. JW Marriott is lovely, but generally pricey, and will likely be very high during the week. The HGI is nice, but I am not in love with neighborhood. There is a Sheraton 4 Points near there, too. Same kind of hotel. Not sure how many of you there are, but I would recommend one of the Embassy Suites hotels, all of which are super. You get a free, cooked to order breakfast, cocktails, and a nice suite with a bedroom and an LR with a sofabed. There is an Embassy Suites downtown, very nice location but a bit business-y. Then there are 2 in the burbs, right next to metro. We are actually staying at the one in Alexandria next week (weekend rate of $139 for 4 of us); if you want me to let you know how it is, let me know. There is another one in Chevy Chase, MD. I have heard that it's the best of the 3, but the prices do reflect that. The advantage to staying in Alexandria would be that on the morning you are leaving, you are minutes away from 95 South, and it is much easier to navigate into and out of than DC (not that DC is that hard...).
4. Finally, you may want to try Priceline. Start with biddingfortravel.com to get educated if you have not used it before. There is a very good list of DC hotels. I have not P-lined in DC but have in NYC and got a $250 hotel for $98 mid-week.
Let me know if I can be of any additional help.