Wow, what a blast! The Denny Creek Slippery Slab is a natural water slide - a sheer rock formation covering a tenth of a mile or so with water flowing through it that creates an amazing water slide for kids (apparently adults too). At this time of year, the water is so shallow it doesn't pose a danger, and the rock is surprisingly comfortable - I damn near fell asleep after lunch.
Listed in both 55 Hikes Around Snoqualmie and Best Hikes for Kids in Western Washington, the water slide is located about 1.25 miles from the trailhead, which is just past the Denny Creek Campground off I-90 exit 47. It's listed as a 500 foot elevation gain, but frankly it feels steeper than that. Although it doesn't feel like a full mile and a quarter... more like just under a mile.
The trail is interesting. It's well groomed, well travelled (the parking was packed today, and runs under I-90 which is an interesting twist on nature. It crosses Denny Creek, and runs alongside it most of the way.
Definitely this is Spencer and Garrett's favorite hike so far! We spent a good two and a half hours there and would have spent more if Spencer had his way. Despite all the people, it really didn't feel crowded - plenty of space to spreah out even if there were 100 or so people there.
A great visit on a 90+ degree day in the Pass.