I did this hike a couple of years back for my birthday with my mom and dog, and despite how inexperienced I was at the time with hiking, it is still my all-time favorite trail!
The Ice Lake Basin hike in Colorado is a strenuous 9.5 mile round trip hike where the 3113 ft elevation gain to over 12000 ft above sea level comes heavily into play.
We started it at -16 degrees Fahrenheit and ended with a balmy 32 degrees Fahrenheit in the middle of October. With more switchbacks than you can count, I ended up having to carry my 20 lb dog, a 20 lb pack, and my mom's water bottle(s) around mile 4 over ice streams and straight up a rocky, slightly questionable mountainside with a steep drop off down a frozen waterfall. (This is not a very dog-friendly hike so don't make the same mistake I made but if you do decide to bring your furbaby, be sure he has time to acclimate to the elevation. We were in colorado for 3/4 days before attempting this and it was still tough).
With all that said and the fact that I could barely walk the next day, I'll be the first to claim that if you don't do this hike at least once in your lifetime, you are truly missing out. The views along the entire thing are stunning! The end scene at the glacier lakes is crystal clear with a blue hue that you very rarely see in the US. The air is fresh and the fir trees intermingle with wildflowers to remind you that certain locations just can't be described... only visited.
if I had one place to visit again and again, it would be the ice lake basin hike.