One of my favorites is Houston Rd. It's easy to bike to from home, and it leads to other gravel roads like Euclid Rd (see Version 2: Trail 100/Houston Rd/Old Trails/Euclid/Trail 25 and more), paved roads, Palisades Park, and you can loop back to Riverside State Park (also see Version 1).
It starts out off Government Way at the same intersection for the Centennial Trail parking lot by the Military Cemetery. Just about a mile long, its a good climb with the grade varying widely, topping out at 14% give or take. It's a good workout.
Here's the garmin map for Houston Road
A short but good climb is Downriver Drive, riding up past the golf course to Northwest Blvd. If I remember right, the grade tops out steeper than Pettet Drive. It's a good one to to take on the way home from work, maybe riding back down and up for some hillpeats. I've never actually done that, of course.
Here's the garmin for Downriver starting where Downriver Dr splits off with Aubrey White Parkway, up to H Street just below NW Blvd.
0.52 mi 166 ft
Elev Gain Distance
Living on the North Side, I don't get to High Drive often but it's like the granddaddy of Spokane Hills. Long and high with good views, more intimidating than it actually is. Earlier this spring I tackled the whole thing, starting at 5th and Cedar. Cedar leads to the start of High Drive, which conveniently ends at The Rocket Market on 43rd where you can re-fuel and re-charge. Overall it's not a real steep climb, but I remember Cedar being a good workout.
Cedar Street/High Drive garmin