Okay I am from this area and DC is NOT far from Tysons unless you are driving in Rush hour traffic. Plus...during rush hour who actually goes INTO dc? Everyone will be going out, towards MD/VA so it really shouldn't be a problem.
I would DEFINETELY hit up DC for some good boutique shopping.
New stores that JUST opened in DC: Vineyard Vines, Nanette Lepore, Juicy Couture.
for shoes - Hu's Shoes. Best place ever! You will find many unique pairs of designer shoes here so you definetely have to visit just to see the variety.
Other great boutiques:
Wink, Blush, Intermix (my fave!!), BCBG, Barney's Co-op, Betsy Fisher, Propper Topper (they sell hats, it's been getting rave reviews this summer)
Here's another tip - Washington Post.com's Fashion and Beauty Section always has great articles about stores and shopping DC. Sign up for the DC Scout Newsletter to be emailed to you 2x a week so you can familiarize yourself with DC shopping - it's such a great thing to have..I love reading it and knowing when/where the sales are! Also read "Trendspotter" from Wash Post as well, they are always talking about hot new items in the DC area and where to get them. You will get tons of inspiration! The Fashion and Beauty page also has a search box so you can look up any of the stores that other members mentioned in this post :)
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You won't get a lot of boutique style shopping in Tysons I and II...it's mostly the "mall" stores (both upscale and normal) like Forever 21, Coach, Abercrombie, Bloomingdales. However you will also find Chanel, Hermes, Gucci, Louis Vuitton, Neiman Marcus, etc.
GOOD LUCK and PM if you need anything. I go to Tysons I/II like 2x a week and DC at least 1x a week. I also do a lot of shopping in Bethesda, MD so if you have any questions feel free to ask.