The corridor of I-90 over Snoqualmie Pass is known as the Mountains To Sound Greenway , and offers many sightseeing and exploration opportunities, only a short distance from Seattle. You can print out and take along a PDF "Geo-Tour" of the MTSG at 
With four alpine base areas summit  ski slopes that vary from beginner to advanced. Nights skiing is also available
Summit Central Has a large terrain park and a halfpipe.
Alpental Offers everything from beginner to deadly (quite literally) terrain. The in-bounds area at Alpental contains a several-hundred foot tall cliff that's pretty great to ski off of, but check the landing for plenty of snow first, and do this only if you're an expert skier. Pay attention to, and believe the double black diamond signs. The Alpental backcountry is quite impressive and readily accessible; make sure to obtain a Backcountry Pass from the Ski Patrol (at their Shack atop Chair 2) before exploring. Drive east on Interstate 90 from Seattle, approximately 1 hour. The road is an active interstate highway, and is therefore plowed and sanded all winter long, but snow tires or chains may be required.
There are numerous hiking opportunities around the Snoqualmie Pass area. This is not an area that much solitude will be found on the hike.
Snow Lake A good hike from July to early October. A moderately strenuous hike, trailhead can be reached parking at the Alpental ski resort parking lot (A Northwest Forest Pass may be necessary). Traverses through forest, reaches an Alpine Lake. Stay and relax, consider a swim. Continue along the trail, go to Gem Lake, and beyond.