If you enjoy architecture and touring older and historic beautiful buildings, I highly recommend Doors Open Denver. Basically, a bunch of older and historic buildings around Denver are opening their doors today and tomorrow for people to just come in and browse around. Some of them also have volunteers working that will tell you a bit about the building's history.
Here is the web link to the official site and guide to which buildings are open:
http://www.denvergov.org/doorsopendenver/template318831.asp Oh, and its all free.
I visited the Governors Mansion with its ornate furniture and decor, the Malo Mansion (which had amazing painted ceilings and ornate woodwork throughout the lower floors), and the John Porter House, which had cool brick architecture and a beautiful tile roof made of green Italian slate.
Update! I added pictures - most didn't turn out, but here are some that did:
The green roof of the Porter House
Outside of the Malo mansion
Close up of the decoration of the underside of the roof overhang
One of the ceilings in the Malo mansion. The entire downstairs was painted in different patterns
The boardroom/dining room at the Malo mansion
Outside of the governors mansion
One of the sitting rooms at the governors mansion
Another pic of the same room
The bar / old billiards room at the governors mansion