Flattop Mountain in Rocky Mountain National Park is one of my favorite hikes. It's good length clocking in just over 9 miles round trip and comes with a good elevation gain just under 3,000 feet. I usually use this hike to restart the hiking season as it takes you from around 9,400 feet up to just over 12,300 feet.
For me, and my skill level I rate this hike as a moderate hike for those comfortable with long hikes that will take you above tree line, this usually comes in around 7 hours for me. I tend to stop a lot and take a lot of pictures. If you time the weather right, it is very peaceful up on top, otherwise it can be very windy on the top. On a clear day the sky is a perfect view and the views of the park are breathtaking.
This year there was a lot of snow on the trail which made the trail a bit more challenging. It's not uncommon to see Marmot, or more specifically have Marmot pose for pictures, they are hams. They must know they are getting their picture taken as they tend to stay still until you have taken your picture. Checkout my GPS tracks here.