Mt. Sai is a wonderful hike (though a bit boring). It goes through endless rounds of forests via rock stairs and soft dirt - up, up, up. And, then, bam! A viewpoint. Other than a few water features, Mt. Sai can be a bit repetitive. But, it makes up for it in cardio. The trail can be grueling and really gives you the opportunity to push yourself. Lots of trail runners can be seen here!
Be aware - very popular trail! You won't get shy wildlife or solitude on this hike.
I strongly strongly strong recommend taking the Transit-to-Trails bus that King County Metro runs. This trail-head parking lot is pretty packed no matter the time of day. I parked at North Bend Transit Center and took the bus from there.