Many people ask what's the best time of year to visit Estes Park for ATV or Jeep rentals. Honestly, every time of year has it's advantages.
Personally, I like Sept - late Oct. There are fewer people, the elk are in rut and running all over town, and the weather is perfect.
Summer is obviously very busy, as Estes Park gets around 4 million visitors a years. However, the weather is beautiful. It's never too hot or too cold. I always recommend bringing a jacket though. Nights do get cool.
Winter is fun because you nearly have the whole town and hiking and ATV trails to yourself. If you've never ridden an ATV (Razor) in the snow, it's something you must try. It's a blast! We also have sleds and Jeep rentals available. Come with winter clothing though. If you don't have any, we do rent winter snow gear.
Spring is an odd time of year in Estes Park. One day it could be 60 degrees and sunny, the next it could be a full on blizzard. Makes it interesting to say the least. I'd come prepared for anything. Even if it does snow, it's usually fairly warm, which makes for a really fun day on the trail.
Colorado has more sunny days than any other state in the country. It's beautiful here.