This did a good job of getting me out of the office early on Monday's last year, so I'm doing it again. Monday rides at Marshal Mesa.
I usually like to ride over to Flatirons Vista South and Dowdy Draw for about a ride that's about an hour to an hour and a half, but we can always determine our desired route/distance based on who shows up and maybe the weather/daylight.
Meet at the Marshal Mesa trailhead just east of the intersection of Highway 93 and Marshall Road. This is one of the easiest trails in the Boulder area; good for beginners, but the network is long enough that you could still get in a 2 hour ride if you want.
I know I won't be free every Monday, and will either need to cancel a few, or just have the group meet without me, but we'll play that by ear.
Here's a map: