I have a few suggestions. There are always the old chain standbys, such as the Outback or Hops at Denver West, but I'm sure that you would prefer something unique.
One of my favorites- and it is very special is the Table Mountain Inn.
http://www.tablemountaininn.com/There is always El Rancho up I-70 in Evergreen
http://www.historicelrancho.com/or Simms Landing at 6th and Simms/Union in Lakewood. They have a wonderful view of downtown Denver, especially at night.
http://www.simmslandingrestaurant.com/A friend of mine raved about an Italian restaurant called Abrusci's at 3244 Youngfield in Lakewood/Wheat Ridge. It is in a strip mall and Applejack Liquors is around the corner. My daughter has also tried it and said that it is really good.
http://www.abruscis.com/contact.shtmledited to add the Heritage Square Opera House up at Heritage Square
http://www.hsmusichall.com/shows.htmThey just finished with their popular Loud show and are showing from Rags to Riches now. They have a buffet, which is decent food, but their shows are so fun! They'll have you laughing and possibly singing along with them!
AND there is the class Briarwood in Golden- very pricey, but extremely elegant.
http://www.thebriarwoodinn.com/ It is probably too early for their elaborate Christmas decorations, but it is a beautiful place that serves wonderful food.
I will add some more, if I think of them. Have a great visit with her!
Dawn
edited to add one more- and I had better stop thinking, as I am getting hungry for some good food. If you don't want to come down towards the metro area, a classic in a beautiful area, although quite expensive is the Greenbriar Inn.
http://greenbriarinn.com/dinner/entrees I haven't eaten there for years, but it is really nice.