There’s tons to go at. Several clubs running from Seattle, have a google for Seattle Colonnade, a bike skills park under the I-5 freeway. Also clubs and trails at Bellingham and Whatcom counties worth googling. I found some nice trails over on Bainbridge Island as well.
There are some incredible north shore trails near Port Angeles on the Olympic peninsula as well, I think THIS is one of them. A lot of the woodlands in from the coastal area is either National Park or State Forest, so MTB is pretty restricted, but with the size of WA there’s still more in the state than in the whole of the UK. If you feel like killing yourself you could ride a road bike up and down the Mt Baker Highway, that’s a grand day out.