I lived in Seattle for a couple of years. I don't often disagree with Tony/Mark...but I do on this occasion.
Lift served skiing. Almost everything that is local to Seattle will close down by May 1st.
However, this looks like a great year, so..........
Crystal Mt. (and the same good people who own Brighton/Big Sky/Michigan Resorts) -- would reopen the mountain for an early summer season. This happened twice while living there -- 98/99 and 99/00. The mountain closed in late April/early May. It then reopened in June through July for a summer season. Almost all runs off of Green Valley Quad and some off of the Rainier Express. Crystal faces mostly East so get there early for less sun damage.
Just monitor it. Crystal would rarely break the 100" mark because it is in the snow shadow of Rainier. This year...they passed it pretty early.
Backcountry.
A very easy - no death trip - for just a mellow corn snow experience... Mt Rainier - Paradise. This about 2.5 hrs from Seattle. However, you could bring skis, hike fore 2.5 hrs and ski nice corn run on the Muir Snowfield ( a snowfield is a body of snow that does not move so no crevasses like a glacier). Corn ripens nicely while you hike on a south face. Do not ski on glaciers -- still waiting for some bods to be spit out. Lots of people around.
Another backountry. Alpental. A great great expert mountain all winter -- very skiable in June. The best skiing is anywhere off the routes around the trail International. There is mor backountry, but cliff hazards are real and sever. The lower you go, the more likely you will cliff out with an 80 footer. Medium/low people around. See Alpental.com for a good map.