Good grief - south seatac?? From Bellingham to ride?
Seatac's got nothing whatsoever over Bellingham, Anacortes, Mt Vernon etc.
Start with Galbraith Mt. Many people call this North Shore "lite" kind of riding. You can find stunts, singletrack etc.
http://www.galbraithmt.com/
There's also Chuckanut Mountain and... crap, can't remember the name of the other place now. Same general area. (Edit: Blanchard Mt.)
You can also drive about 45 mins south to Anacortes where there's about 30-40 miles of incredible singletrack in the Anacortes Community Forest Lands. (I used to live on a street with a trailhead and within a half mile of 3 or 4 others - GREAT riding there!)
Then there's the "Tree Farm" in Mount Vernon. Possibly the best section of singletrack I've ridden... after a nice climb to the top, you come down this section - not very long, but smooth, swoopy, through the trees and fast. Just puts a huge grin on your face and makes you want to climb back to do it again and again. That's probably 30-40 mins south of B'ham.
There's also the Walker Valley ORV area. I've only ridden there once and it was pretty fun. Some of the trails were pretty torn up by the bikes, but others were really nice. Went OTB gawking at scenery and bruised a rib when I rode it.
And of course, B'ham's only an hour or so from the North Shore, should you want to venture up there.
Drive to "sst" - at least an hour and a half - instead of riding what's right in your lap? Sorry... no.
Oh yeah, renting bikes? I've rented from Fairhaven. You can demo high end bikes there. They carry (or used to, at least) Specialized and Cannondale that you could demo. There's also Skagit Pedal Sports in Mt. Vernon and Anacortes that do demos, too. Or used to.