First off, hit the U-District and have a few beers and buy yourself some Seattle chronic. Even if you don't smoke, it's good enough there to get some and do it anyways.
Plus the U-District is college central and just rocks in general. Hot girls, good drink specials, awesome live music, the list goes on and on. Definitely spend a night there living that experience.
Also, make sure you spend a night or 3 in Pioneer Square towards downtown. Preferably on the weekend if you can. That place is poppin' and there are over a dozen bars down there with live music, and see all of the above in relation to the U-District. You should easily be able to kill a week at those places alone, but feel free to check out Fremont, Ballard, and the Showbox for some other hot party places at night.
As for touristy stuff, obviously the Space Needle, EMP experience, and the Pacific Science Center should be on the list. Pike Place, and a few other places have been mentioned. Check out Gasworks Park, it's pretty cool too. They're all in the downtown vicinity, but I would personally dedicate an entire day to EMP. PSC and the Space Needle should be able to be accomplished in one day.
From there, check out the Sound Garden (Yes, the band did get their name there), Gasworks park, Greenlake, and the Fremont Troll. That troll is ****ing epic and something most people never come across in their time here, but it's a timeless piece of Seattle hippy, artsy history. It's awesome. Fremont in general is a great place with it's own vibe, and should be experienced fully.
That stuff should keep you plenty busy while you're here, and it's what comes to the top of my head, but there's so much more. You shouldn't spend one bored moment in that city. I already miss it. If I think of anything else that you can't live without, I'll post here again. But I highly suggest freelancing a day or two while you're there and don't make plans. Things will come up as you get familiar with your surroundings.
Oh, and make sure to get yourself some Dick's. Everyone in Seattle loves Dick's. I could go on for days and days about the quality of Seattle's Dick's. It's a staple of the city. My favorite place to find Dick's in Seattle is either in the U-District, or up in Lake City. Highest quality you can find is there.
Have fun man, although it would be hard not too.
And I'm just assuming you're going there on Labor Day weekend for Bumbershoot, so I didn't bother mentioning it. It's an obvious selection.