Have you ridden around Mercer Island? It's not long, but quite fun with the curves on the east side of the island. The I-90 bridge has a dedicated bike lane to get you there.
And have you you tried anything on the west side of Puget Sound? Take a ferry to Bainbridge Island (ferry from downtown Seattle) or Vashon Island (ferry from West Seattle; lots of hills there) for some great rides.
Snoqualmie Valley east of Bellevue has a beautiful, generally quiet road on the west side that we really enjoy. There's a flat gravel trail on the east side of the valley that we rode a few weeks ago. Nothing challenging (former railway) but quite pleasant with big tires.