Cub Lake trailhead is located in the Moraine Park area of Rocky Mountain National Park.  There is parking available, if you arrive early and there is bus service available too.  Cub Lake is a nice lunch area or follow the trail which circles over to The Pools another nice place for a picnic.  From there, you can either return back to Cub Lake the same way you came, hike up to Fern Lake (which is much longer hike) or follow the trail from The Pools back to a parking lot where there is shuttle bus service available.  However, if you parked in the Cub Lake parking area, it is just a short hike back to your car from the parking lot that you will come to from The Pools and may be quicker than waiting for the shuttle bus.