i'm heading to washington dc with my family in july for 3-4 days. hows the weather and what should i pack to look alright but still comfy? any ideas to see or visit that would appeal to a 15 year old? any good shops or vintage/thrift stores? bookstores?

Check the weather websites before you come. It will probably be hot and humid.

Check a guidebook to see what appeals to you.

Or at least do a search on this board. There are hundreds of threads that contain information on hotels, restaurants and sightseeing, even in particular sightseeing with a teenager, that would be of help to you.

For a good bookstore, hit Kramer's in Dupont.

Very, very hot - and very, very crowded. Definitely do one of the Trolley tours (tons of companies that do them - good way to see all of downtown). There are so many great things that would appeal to a 15 year old - you have to be alot more specific about what your family likes.......

As others have mentioned, it will likely be very hot and humid. DC is generally not a super fashion conscious city, so don't worry too much about what you're wearing (if I had a dollar for every fanny-packed family wearing matching just-purchased "FBI" t-shirts that I see during the summer I would be very rich ) I wear mostly tank or sleeveless tops and skirts or sundresses when I am out and about in the summer.

I second the recommendation for Kramerbooks in Dupont Circle. Also in the Dupont area is a fabulous little used bookstore, Books for America (on 22nd St. at P, across from the gas station) - it's a small place but has a great selection with books stacked floor to ceiling, and dirt cheap prices.

For vintage, try The Remix on Capitol Hill (right across from the Eastern Market metro stop): http://www.remixvintage.com/index.php. If you're there on a weekend, you may also want to check out the craft/flea market nearby at Eastern Market.

As for what to see and visit, it is a little hard to make a recommendation without some additional information on likes/dislikes, etc., since what I would consider "must-sees" might not appeal to you. Also, if you provide a little more info on what you're looking for shopping wise, I'd probably be able to make some additional suggestions there as well - I am a champion shopper.