We hiked the Flat Top Mountain trail, starting quite early in the morning from the Bear Lake trailhead.
This was recommended by a Ranger after we said we wanted a fairly tough hike.
It is an 8.8 mile round trip and also 3000 feet of climbing (and then 3000 feet of descending which I found tougher). It goes over 12'000 feet, so if you aren't used to altitude this can be tough.
There were some lovely views and lake overlooks. As you ascended the vegetation around changed from lots of Aspen by the lake, then more firs, then small shrubby trees until at the top there is very little at all, other than some patches of snow and a little glacier.
We also saw some nice wildlife, Marmots, Deer and Pika.
The round trip took us 5 hours and 20 minutes, taking plenty of photos, admiring the views and pausing for food / drink.
We would recommend proper sturdy footwear and to come equipped for bad weather conditions if it the weather changes while you are on the mountain.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.