In 2002, I lived in Colorado, in Northglenn. The first mountain I climbed in Colorado was Flagstaff Mountain. I'd been to Colorado before, when I was a little kid, but didn't remember it.
From behind my house, I could see Flagstaff, Green Mountain, and the rest of the Boulder Mountain group very clearly. They were beckoning me to climb them!
The first time I climbed in this area, I took Route 36. Route 36 comes to the top of a hill, with a viewing area to the side. Here you see the majesty of the Front Range in front of you. The entire Boulder Mountain Group with Longs Peak and the Indian Peaks hovering high above!
As for Flagstaff, I climbed that first. Took baseline road, and drove straight towards Green Mountain, to the (free) parking area on the left at Chautauqua park. From here you can see hills start to rise, and then the dramatic 2,500' prominence of Green Mountain and the Flatirons. To the right is Flagstaff Mountain.
There are many places to rock climb on Flagstaff Mountain, and a steep, fun trail, that crosses the road a few times. The first time I did it, I took shortcuts, by climbing rocks up to the summit.
Of course, there is a summit area with an amphitheater, and then beyond that, and up the road a bit, is a higher prominence with rock cliffs, and superior views of the Indian Peaks. I was there when my parents visited, and my Mom freaked out when I got too close to the cliff edge.
Green Mountain is always a fun climb. You gain a lot of elevation quickly without realizing it. It is the easiest 2,500' I have ever climbed.
The flatirons are also fun, and there are lots of trails to hike in that area as well. You can get pretty high without actually climbing the flatirons.
Off of route 170 you'll find the South Mesa Trailhead, where you can climb S. Boulder Peak and Bear Peak. This hike starts off with about 2.5 miles with minimal gain (like 600') and then you tail off into the woods where you climb nearly 2,000' in a mile. When you finally reach the top of the woods, you come out on the ridge, where you can take a left towards S. Boulder Peak, and a right towards Bear Peak.
Bear Peak is more fun, because it is very rocky (more reminiscent of a 14er) and there is some actual rock climbing at the very end of the hike. Bear Peak has great views, but always seemed to have more insects than S. Boulder Peak (just 3/4 mile in the other direction).
I have even hiked in El Dorado Canyon a little bit (which is at the end of route 170). The climbing there is excellent, while the hiking is pretty nice as well, although nowhere near as interesting as Green Mountain or the Boulder Peaks.
All told, I climbed these mountains many times, exploring them, running on the trails, slipping a little on the ice in early April, and talking with other hikers about how to climb the snow-capped peaks further out.
These are great peaks to start off on to acclimatize to Colorado hiking. They are steep, with moderate length trails.
Of course, back in 2002, I didn't have summitpost to direct me on how to get to the Colorado mountains, until all these wonderful guys and gals put up pages for the Indian Peaks and so on.
That being said, the Boulder Mountain group has a lot to offer, and I climbed these peaks more than any other (except Huff-Puff in Montana).