Any day before or after the RPM:

9 am to 5 pm: Visit the Colorado Railroad Museum (closed on Mondays)

Railfan North Yard in Denver (they kick cars up the ladder on some occasions)

On Friday:

Colorado Live Steamers have an event in Byers, CO

10 am to 6 pm: Visit the Rocky Mountain Train Supply store

On Sunday:

9 am to 2 pm: Open house / layout visits

9 am to 4 pm: Rails in the Rockies Trains Show in Estes Park