For those wanting more experience with gravel, Gus Jansson and I just did a 60 mile ride on mixed roads (paved and unpaved), going from Woodinville to Sultan and back. Lots of gravel, most of it quite rideable. Here's Gus's RideWithGPS track, complete with pictures:
Gus and I used our randonneur bikes with 42mm tires. I would think 35mm tires would be fine, 32mm would be the minimum.
The ride was 6 hours of cycling and 2 hours of stops--some elegant, like the coffee stop at the winery just outside Woodinville, some inconvenient, like the stop to change a flat that occurred just as we were transitioning from gravel back to pavement south of Sultan.
The ride is just the middle section of a longer ride reported on Jan Heine's website, where Heine and friends cycled from Seattle to Sultan to Lake Spada and returned in one day, 140 miles round trip:
That is very impressive cycling indeed.
If you just want to pick up a little gravel riding with some challenging but short climbs, you could ride the portion of the Tolt Pipeline Trail that heads East from Duvall.
Does anyone else have a favorite gravel or mixed gravel/paved ride they could share?