This one is a fairly easy trail to the Mt Sanitas peak that avoids the crowds and still offers the views.
This is a mix of trails that will bring you on an easy loop to the peak of Mt Sanitas, but will keep you away from the usual crowds of the more well-known paths up. The hike is a clear path that brings you 5+ miles from around 5,500 ft in elevation to 6,800.
You'll start at the Centennial Trailhead, which offers free parking, and focus on the Sunshine Canyon Trail going west up the canyon. You won't see or hear it much, but you'll be moving up the South side of Sunshine Canyon Drive.
In about 1.3 miles, you'll arrive at a private driveway. This is where your trail ends, and you'll head out the driveway to take a left up the road. Be careful of the cars, as they tend to move quickly here. You'll walk about 40 feet, cross the road, and find Lion's Lair Trail.
The next 2.7 miles will bring you gradually up to some beautiful views of Boulder and the surrounding mountains. If you'd prefer to stay away from crowds, take your moments and pictures on this trail. Once you get to Mt Sanitas you'll almost certainly be sharing the trail.
Once you hear people talking at the peak, you'll know you're close to the summit.
From here, you'll choose your way down Mt Sanitas. The shorter route is about 1.3 miles, is pretty straight, and will drop you just East of your car.