Candidpicker, it depends on how much time you have, but if your just talking in the greater Seattle area, there is plenty.
All good advice above. For music stores there is:
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The Guitar Store- PRS dealer
8300 Aurora Ave N
Seattle, WA 98103
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Emerald City Guitars- vintage
83 South Washington St
Seattle, WA 98104
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Restaurants
Ivars Acres of Clams in Seattle on the waterfront
1001 Alaskan Way
Seattle, WA 98101
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Elliot's Oyster House
1201 Alaskan Way, pier 56
Seattle, WA 98101
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Sights
Go up Queen Anne hill to Kerry Park
Great view of downtown Seattle with Mt. Rainier as a backdrop. Great for pictures.
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As others have said
Pike Place Market
Between 2nd Ave and Western Ave
On Pike Street
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Just a block or 2 away from the market is the waterfront. Lots of construction going on right now and parking is at a premium.
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A little ways out from Seattle:
Jimi Hendrix grave site
350 Monroe Ave NE
Greenwood Memorial Park
Renton, WA 98056
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Also kinda touristy trapish, but you could also check out Experience Music project (EMP) or Museum of pop culture (MoPOP) what they call it now.
410 Thomas St
Seattle, WA 98109
Seattle based museum with some good galleries including pop culture, cinema, sci-fi and of course music.
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Weather is nice this week with clear blue skies and sunny, but temps in the low 40's, cold for Seattleites. Of course, it is customary to tell anyone thinking of coming to Seattle that it rains all the time, that way no one wants to visit or move here, but I think people are figuring out that we lie about that....a lot!
Good luck and have fun!