i also recommend lake serene - a nice short hike just to the waterfall and a bit farther to the lake, and an easy way to introduce the non-outdoorsy some local beauty.
also recommend the north cascades hwy - ooh, aah.
also recommend even driving up to the pass (snoq. or stevens) or all the way out to l-worth if you're into that sort of thing.
if you don't mind a longer drive and maybe a nice ferry ride, the spit recommended above is a really nice way to see some of the more coastal part of wa. the Lake Ozette/Cape Alava/Sand Point Loop hike out on the coast is very cool - longish but very flat. would probably be pretty chilly this time of year, but walking on beaches is always nice and the ferry ride might afford some good views, too.