I am little confused as to what you mean by Seattle DC. Are you getting mixed up over the fact that Seattle is in Washington State (On the pacific coast) and Washington (The capital and on the Atlantic Coast).
If you let me know which I am fairly familiar with DC and My wife is from Seattle so know it very well. about 6 years ago
In terms of places to visit it really depends what you are into but hopefully I have covered the main items below plus a few sneaky secrets.
1) Pike Place Market - The main farmers and artisan market is located on 1st Ave and Pike. For me it is the heart of the city. It is also where the famous fish market is located showing the days fresh catch as well as the famous flying fish display. There are also a lot of small independent artists shops some great little cafes, street food and restaurants in the warren of passages.
2) Seattle Centre - Home of the Pacific Science museum, Children’s museum (both good if you have children). The SIFY museum, Experience Music Project. Also home, of course to the Space Needle the most iconic landmark in Seattle, take an elevator trip to the top for a panoramic view of the city. To get here from the city centre take the monorail from the West Lake Centre
Tip: Most people rush to the Space needle thinking it the best view. It isn’t. The tallest building in Seattle is the Columbia Centre (Located at 4th and Columbia). You can take a ride to the observation desk for around $5 and see the whole city as well as the Space Needle.
Restaurants: My favorite is called 5 Spot and is located in the Queen Anne District (at the top of the counter balance). This has a 3 month rotating menu featuring regional dishes from around the US. Excellent service and food and a very laid back atmosphere with the restaurant decorated according to the menu theme. You will need a taxi from downtown and I recommend a reservation.
Also check out Pike Place Brewery, good artistically beers burgers, steak etc. As well as Pike Place Chowder.
If you have the time I fully recommend taking a trip out to Mount Rainer, (the dormant volcano which towers over the Seattle), however it will take a whole day.
Hope this helps if you need anything more just let me know. about 6 years ago