It is a little confusing to find the trail to Mystery Lake. There are signs but it is not very well marked. You could make this hike longer by starting after the washrooms, past the sign. Walk to the Dog Mtn trail and just to your left is an old skinny creek bed and you can start walking up here, eventually you will come to an old logging road. Go left and stay on this road. Do not veer right up the other rocky road (unless you want to go up by the chairlifts...it is pretty but you will have to come back down the unstable rocks). keep going straight along the road and there is a sign, follow the trail up and around and soon you will come to the lake. We walked to the left and around it and came to an open area where people were sitting and lots were swimming. It was pretty. Right behind that area was a trail going down. After we relaxed a bit we headed down this un-marked trail which was a nice hike back down to the parking lot. The trail down is rooty and rocky an comes out just across from Dog Mountain TH making it a loop.
If you want to hike to Mystery Lake an easier way, head past the washrooms as if you are going to Dog Mtn, across the road is the trail by the chairlift. There is one sign post and it looks as if nothing is on it but on the other side it says Mystery Lake 1.2 km's. Go up this dirt, rocky, rooty trail that has no signs the entire way up. There are orange trail markers though. It is pretty and a fun trail that is not scary at all. It has some rocks and tree roots to step over but nothing challenging. You will know you are on the right trail as you will walk along a couple of wooden planks, and will see a small pond on your right. At one point you will come to an open valley area that overlooks the city with tons of greenery. It took my breath away. Keep going up this trail and soon you will come to an opening and there it is, Mystery Lake. That is the easiest way in & out. It is a great spot for lunch and swimming. You can also walk around the lake and check out some of the other trails. Happy & safe trails!