There are lots of good hills around. You can find any degree of difficulty you want. For a steady climb in the city, try 8th Ave NW from Leary Way (near the ship canal) up to about 85th St. It's no too steep or long and it has a bike lane most of the way.

Another option is to start near Montlake Playfield and go southeast up Boyer Ave to 24th Ave NE, then go right one block and right again on Interlaken Ave. Then follow Interlaken up to the intersection of 19th Ave NE and East Galer St. From there you can go up 19th a couple of blocks and turn right toward Volunteer Park on top of Capitol Hill. It's a very nice ride.

There are lots of hills to choose from on this ride: http://home.comcast.net/~cheg01/waterworks1.htm
though they tend to be pretty steep and some of it is on high traffic roads

Other good resources:
http://www.bikely.com/listpaths/srchkey/seattle
http://www.routeslip.com/discover/USA/WA/Seattle
(routeslip has distance and elevation)