Bear Lake Trail
This is a popular trail, which is beautiful any time of year, although busiest in the summer. The aspens turn in late September/early October.
Time of Day Tip: During peak summer months, the parking lot of this popular trail is already full between 8:30 and 9 AM. If possible, be there early around 7 AM.
We were there on October 12th, which was a very good balance for us: Late enough to avoid the summer crowds and to see some snow, yet early enough to see some of the aspens with their yellow leaves and it wasn't too cold or difficult to hike the trails. I think the Rockies are always beautiful, but I believe they're most beautiful with some snow. To catch the most aspen color, I recommend going in late September or early October.
In my opinion, this hike gives you the most "bang for your buck" in Rocky Mountain National Park: You can see four beautiful lakes in under 2 miles (or under 3 km) (one way) and a gain of elevation of 605 feet (184.4 meters). If you go all the way around Bear lake, then all the way to Emerald lake at the end of the trail and back, the total distance is about 4.2 miles (or 6.8 km). It's 0.6 mile (1 km) all the way around Bear Lake, then a 0.5-mile (0.8-km) hike and 225-foot (69-meter) gain in elevation to Nymph Lake, then a 0.6-mile (1-km) hike and 200-foot (61-meter) gain in elevation to Dream Lake, then a 0.7-mile (1.1-km) hike and 180-foot (55-meter) gain in elevation to Emerald Lake.
We wore several layers of clothing, and as the trail went in and out of the sun, we removed and then reapplied layers as needed. As we arrived at the highest point at Emerald Lake, it was cold enough to require all layers.
We were in Colorado for about a week, and this was my favorite hike on that trip.
For more information, go to http://www.nps.gov/romo/index.htm