Animal instincts Thurday-Dec. 31. Time off during the holidays means relaxation for many, but also activity, especially if you’re trying to keep the little ones occupied. Thankfully, the Denver Zoo is offering free admission through Dec. 31 for those 15 and younger. The City Park institution, which glows this time of year with its nighttime Zoo Lights installation, features 2,500 animals across 80 acres, along with daily animal talks and demonstrations featuring Asian elephants, African penguins, bighorn sheep, flamingos, vampire bats, and tigers. The zoo is open 10 a.m.