I will be in Denver in December. What are some Christmas time must for the area?
I would suggest Larimer Square as well, and the lighting of the city building, you really can't miss it! It's very festive. Also Denver Zoo lights are fun and the the Denver Botanic Gardens trail of Blossoms is pretty too. It's even more festive and fantastic when it's snowing out. There is also the Christkindle German market at Skyline Park, it's small but interesting.
You may want to try and head toward the mtns. if you can. Georgetown (Colorado) is very pretty and then of course Vail, which is one of my favorite places. All this stuff you can google and figure out what you would want to do! Have fun!
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In general, the lights and atmosphere of Larimer Square are a lot of fun, please the restaurants in the block are amazing. Lights at the capital building are also worth the visit.