You'll love Safeco - it's a beautiful stadium. With regards to you question about what to do, what type of interests do you have and how far do you want to travel? I live in Tacoma (about 45 min south of Seattle) and we have a few musuems: Musuem of Glass (glass and other art, not windows, with a cool glass blowing demonstration), Washington State History Museum, Tacoma Art Musem. They're all w/in walking distance of each other. And I think some of them are free on Thurs nights.
There's a 'scenic railroad' that goes to Mt Rainier. I've heard it's quite fun. If you like to hike, etc, there are hundreds of places to go, besides just Mt. Rainier - though you must go see Mt Rainier up close and personal. The Olympic Nat'l Forest has a rainforest, as well as many great hikes and stunning lakes. Washington is known for it's outdoor activities. If this is what you're hoping to do, stop by the REI flagship store in Seattle. It's an amazing store, and people there can make suggestions as to what/where to go. Kayaking, rock climbing, hikes, etc.
If you want to go a bit north, you can ride a ferry to the San Juan Islands. They do orca whale watching in the summer. The islands themselves aren't too terribly exciting, but they're beautiful.
Head southwest and hit the coast (pacific ocean). Head east and hit the mountains. Lots of wineries in Washington. A few N of Seattle (Chateau St Michele, located in the town of Woodinville, which also houses the Red Hook brewery), but many are in E Washington. Check out the Gorge at George Amphitheater - there might be a show you want to see while you're here. It's one of the nation's best outdoor ampitheaters.
Portland is a fantastic city. I hate to admit it, but in many ways it's better than Seattle. As was suggested, stop off at Mt St Helens on your way. It might be your last chance to see it as is, as it's had quite a bit of volcanic activity lately.
Hope this helps. Enjoy your visit!
Rebecca
