Not necessarily layouts, but here is a list of places to go for those of us with railroad interests:
With a few hours available:
Caboose Hobbies - One of the biggest.
http://www.caboosehobbies.com/
Forney Museum - Home of Big Boy 4005
http://www.forneymuseum.com/
Colorado Railroad Museum - Better every year, 20 minutes west of Denver.
http://coloradorailroadmuseum.org/
The Georgetown Loop - At Georgetown/Silverplume. 45 minutes west of Denver.
http://www.georgetownlooprr.com/
With a day available:
Royal Gorge Route Railroad - Southwest of Colorado Springs. 2 hours from Denver.
http://www.royalgorgeroute.com/
Leadville, Colorado & Southern Railroad - 2 hours west of Denver.
http://www.leadville-train.com/
Pikes Peak Cog Railway - West of Colorado Springs. 1 ½ hours from Denver.
http://cograilway.com/
With a couple of days (minimum) available:
Rio Grande Scenic Railroad - Standard Gauge Steam and Diesel excurisions from Alamosa or La Veta.
https://www.coloradotrain.com/
Cumbres & Toltec Scenic Railroad - Southwest Colorado, all day ride one-way (motor coach return same day). Start from either Antonito, CO (4 hours from Denver) or Chama NM (6 hours from Denver).
http://www.cumbrestoltec.com/
Durango & Silverton Narrow Gauge Railroad - Southwest Colorado, all day round-trip ride. 6 hours from Denver.
http://www.durangotrain.com/
Have fun!!