Mariposa Grove in Yosemite National Park has a series of trails which leads you through massive ancient Sequoia groves. You'll pass the Grizzly Giant and if you hike in about 3 miles, you can reach the fallen tunnel tree. Mariposa Grove is renowned for their giant ancient Sequoias and this trail is likely one you should not skip out on when visiting Yosemite.
Round trip to the fallen tunnel tree and back is about 6 miles, depending on the route you take. On this GPS map, the fallen tunnel tree is located at the top of the northern loop, just before the spur trail leads out from the loop.
Lookout points: There are no real 'lookout points' above the sequoias, but there are a multitude of the ancient sequoias living and dead - along with the grove of sequoias around the ranger station, which dwarf the building. Consider those as your 'view points' for this trail.
Note: The GPS data provided includes an optional loop, as there are various trails that branch out and loop back, so the total mileage and GPS mileage do not line up one-to-one.