Mount Si via the back way. Begin from the large Mount Teneriffe parking are and add on some miles to sneak up this popular peak from behind.
From the parking area, you'll start out on the Mount Teneriffe trail, a road-to-trail conversion that offers gorgeous lush forest, and a pleasant, almost flat walk for the first 1.5 miles, before getting down to business and climbing in earnest for the next 2.7 miles. Luckily, the views along the way are quite good, so you have plenty of opportunity to stop and enjoy them (while catching your breath, of course).
Notable views through this steep old road include Mount Rainier and Rattlesnake Ledge. Look out for an overlook with a large stone bench, placed there by the contractor who did the road-to-trail conversion for hikers to take a rest.
At a 'T' intersection 4.2 miles from the trailhead, make a left at a junction marked with a DNR-provided sign of the trail network. Turn left, and you'll continue to climb for about 0.4 miles through rolling forest before reaching a high point. Through here, be sure to stop and look up. You'll have a view of Mount Si most hikers never see. On this same section of trail, look north along the Puget Sound coastal plain to see Mount Baker and, on a clear day, all the way to Whistler Mountain in British Columbia.
Soon, you'll find yourself descending the final 0.4 miles to the base of the haystack on Mount Si. This is the official end of the trail, though it's possible to scramble the Haystack. If you choose to do this, do be careful; it's accessible only via a narrow chute on a rock face and can be daunting for even those fairly confident in their scrambling abilities.
From here, you can choose to loop this trail, by descending on the Mount Si trail and traversing back to the trailhead on the Talus Loop or Roaring Creek trails, or you can come out a the Mount Si trailhead if you have a pickup arranged.