The Denver Zoo can be a little pricey if you've got a family in tow, but not on these days. Just a heads up though — with it being free and all — it might be a zoo. 

The Denver Zoo has a handful of 'free days' every year... typically during the colder months, but hey, it is free. During the winter, the Denver Zoo is open 10 a.m. - 5 p.m. every day, and on these days, you can get in sans-admission:

January:

Friday, January 18, and Saturday, January 19

February:

Sunday, February 3 and Monday, February 4

November:

Monday, November 4 and Friday, November 8

More information on hours and pricing here.