Marshal Mesa area kinda sucks IMO. I got there often to trail run, if I ride there it's usually on a CX bike.
Walker Ranch is definitely not intermediate.
I've never been to Rabbit Mountain
Heil is much MUCH different post September 2013 insane flooding. Start at the bottom of Picture Rock, ride it up, to the top loop counter clockwise but stick to the outside, when come to Wapiti ride it down to the lot then turn around ride it back up and ride the rest of the top loop still counter clockwise to back to Picture Rock and back down. Its a fantastic ride that finishes with about 7 miles of downhill. All that loose rock on the top is gone, it used to flow better clockwise but now going counter clockwise you climb while it's smooth and descend through some really fun chunky stuff back to the top of picture rock.
Hall is popular when it's linked with Heil/Picture rock. There is a pretty ridiculous super grawly rock garden (if you can call it that, it's LONG, like minutes of solid rock) beyond that it doesn't seem like anything special. I've only ridden there a few times, it's not a top choice IMO.
Here's a GPS route that shows Heil with the top and Picture Rock 9the top is in the opposite direction, and goes into Hall but it gives you a good idea. http://connect.garmin.com/activity/244262759
Green Mountain is a relatively short nothing special ride but you can link it with Dakota Ridge which in all honestly is far above intermediate.
For staying in town Betasso is super fun with lots of flow and awesome views. The trails are pretty short, you can start from Boulder Canyon and go up the Betasso Link trail which adds some distance and a lot of difficulty. You'll see maybe 1 in 30 that can clear it going up but it's a ton of fun when you have to come back down. I like Betasso but I usually use it as an after work ride. No bikes allowed on Wenesdays and Saturdays, it's 1 way but the direction changes regularly (I prefer clockwise but it's fun no matter which way you ride it) http://connect.garmin.com/activity/369063366
Not much further than Green Mountain there's Lair O The Bear, again a pretty short ride but super fun and flowy out and back. It's nice that the back is more than the out as well.http://connect.garmin.com/activity/595180113
For me I always loved Kettle and Palos was a very VERY distance 2nd. Buffalo Creek blows them both away no contest. It's an IMBA Epic, it drys every well, it's not overly technical (unless you have a death wish, than you can ride BlackJack). On the Epic route everything is ridable, the views are nuts, the scenery changes up a lot, when it's crowded it's so huge that it's not crowded and the flow is second to none. Buffalo Creek tops everything else I've ridden anywhere. Here's a recommended route (basically the Epic route), that ends with like half an hour of flowing almost no pedaling downhill, it leaves you smiling for days. http://connect.garmin.com/activity/600292629
Another IMBA Epic within driving distance (2 hours north just into Wyoming, you can camp on the trails as well) is the Kurt Gowdy State park. It's intermediate with a hand full of difficult features and a lot of fun. I don't have GPS data from it.