East of Seattle in Issaquah, the Grand Ridge Park has a six mile singletrack running from US90 north to Duthie Hill Park. It has about 500 feet of vertical climbing from US90 up to the summit.
To reach the mountain biking trailhead for Grand Ridge Park from Seattle, head east on US90 to Exit #20. Head north on 270th Street briefly and look for trailhead parking. Start riding west on the Issaquah Creek Trail for about 1 mile to the start of the Grand Ridge Park trail.
If you are looking for more action, check out the Duthie Hill MTB Skills Park which is located at the north end of the Grand Ridge Park trail.
Overall, Grand Ridge Park is the 56th most popular mountain bike trail of all 59 mountain biking rides in Washington.