Goldengate Canyon State Park is a great destination close to Boulder and the Front Range.  On the way to Golden from Boulder, make a right (I foret the road but there's a sign for the park), and about 15 miles later, you're in Golden Gate Canyon State Park.  You can get a map at the guard shack ($4 entry fee for a state park) and hike from any one of numerous trailheads.  It's not a big park - if you spend the day there you can hike a good deal of the park and get the idea, but there are many trails if you want to come back. 

We had a great time, and hiked up past an old bulldozer to some old cabins.  There were plenty of wildflowers, and we tried to find a natural spring that was shown on the map, but wandered around for awhile and never saw any water.  Hey - I'm not gonna snow you - this IS a state park. There's people having picnics with big coolers, there's non-hikers hiking around, and there's grills and RVs. But the fact remains that once you head out on the trails, its really, really pretty. It might not be a hard-core backpacker type outing, but it's worth the trip. Dogs are allowed, so if you want a nice hike without having to put up with a long drive or that I-70 traffic, I recommend the park completely. Have fun!