We hiked up to Lake Gibraltar, which included some fun stream crossings. It was like 18 miles round trip, so my knees weren’t very happy. The last mile or so was mostly off trail, which is my favorite sort of hiking. I think it would be fun to try to go cross country from the lake back to a few waterfalls a mile or so back. It's actually one of the flattest hikes in Boulder county with only 1600 ft of elevation gain.