I live up on the Sammamish Plateau (often commute home from UW, nice hilly 25 mile ride).
Lots of good workout climbs in the area:
-Lakemont Ave. up from Newport Way, as Bek said;
-Highlands Drive MUP up from Front Street in downtown Issaquah. Nice, virtually unused MUP from there up to Issaquah Highlands, and you can climb to the top of the Highlands if you want more. Nice view of central Puget Sound from up there (for now; constant addition of big ugly houses is gradually blocking it off). Some people ride up Highlands Drive itself, but it's shoulderless and crammed with far too many kamikaze commuters at 50mph for my taste.
-Duthie Hill road northeast of Issaquah. One of the few hills around here that doesn't have a *&%! stoplight at the bottom so you can really enjoy your downhill speed!
-Fall City to Snoqualmie Falls. Not very steep, but long enough to get the muscles going and there's the lovely Falls at the top. For more workout, continue on east about 1/2 mile to the Snoqualmie Parkway and climb a few hundred more feet through the Snoqualmie Ridge sprawl to the I-90/Highway 18 junction. Great evening ride when the cliffs of Mt. Si are glowing in the last of the sun.
-I've even seen people climbing highway 18 from I-90 up to Tiger Summit. I like brutal climbs, but I'll pass on that one! I'd rather survive my ride, thanks.