Island Lake can make for a full day of adventure packed with beautiful scenery (alpine lake, wildflowers, etc.). It doesn't hurt that there's awesome cliff jumping at the lake as well.
There are many lakes in this area, but Island Lake is the only one with epic cliff jumping. You can camp anywhere around the lake and campfires are permitted.
The cliff is best to jump off mid summer when the water is the warmest, but it's always pretty cold. The cliffs are about 30-50 feet tall. It's only about a 2.5 mile hike from the trailhead and is mostly flat, except for a few switchbacks at the end to get up to the lake.
This is an awesome weekend adventure with all your friends.