Toward the south end of Whidbey Island is Lone Lake (found going north on Bayview Rd off the main hwy). It's a year-round lake.
To further discover lakes on your own, I'd suggest first scouring the regs book for the fly fishing and selective gear lakes that are open. Second, hit the WDFW website to check which lakes were planted with Triploid or otherwise trophy sized fish. In this case, you have to check back with the regs to see if they're open. Next, check the DeLorme's Gazetteer to look for indications that a given lake is overly developed--that is, encircled with the kinds of roads that indicate tons of houses. If the lake is caged-up with streets, avoid it--it's probably been over-fished by the people living in those waterfront houses. If you notice that there is little to no development, have heart. And, above all, be sure to check <www.washingtonlakes.com>. That's a great resource that will help you find the lakes closest to home and then further away. WL.com has a clickable map that's divided up by county. Very useful! Good luck.