Upper Slate Lake via Rock Creek Trailhead is a roughly 8 hour hike each way, with incredible views. This is a traditional Backpacking trip, not just a day hike.
Starting from the Rock Creek Trailhead parking lot, it is an up and down trek to Upper Slate Lake. The trail map states 10 miles each way, but accounting for switchbacks it ends up being closer to 14 long, hard miles.
You will be among aspens and pines for the first half as you have a few moderate elevation gains and losses before you come to the Slate Lake Trail at a clearing. At the old burned down cabin, take a left onto the Slate Lakes Trail. From here you will follow the Slake Creek drainage area though meadows for about 4 miles. Next, you will encounter a sharp elevation gain over the next "1 mile." (Note: contrary to most reports, there isn't a well defined trail down to Lower Slate Lake if you're looking stay there first to break up the hike). The next mile to Upper Slate Lake will gain about 1200 feet and leave you cranky and thinking WHEN DOES THIS THING END?!?!
When it does end..... one of the most beautiful lakes you'll ever see. Nestled under some of the "alphabet peaks" of the Gore Range, the views are mesmerizing. There are plenty of places to camp around the lake.
If you're up for a day hike, follow the lake clockwise, staying high (light trails can be spotted/followed) all the way to the waterfall. Follow the waterfall up a bit and there are a few points to cross over safely. Once in the meadows you can find a nice rock to nap on, or venture back further. From these meadows are where most of the entry points are to summit some of the peaks overlooking the lake.
Even on the hike out, you will still have multiple steep gains, so don't think you'll have a lovely stroll back to the car.