Ben and I tackled one of the Sumas Mountain trails a few Saturdays ago after finding the hike in Ken Wilcox’s Whatcom County. See also Kulshan.
It took us nearly 2¾ hrs to travel the 4½ miles and gain the 2500 feet to the lookout at the top; time down was roughly 2 hrs. It was a sunny day when we were there, so we spent an hour just sitting there at the top. From the lookout points, you can look down over the plains and into Canada.
We saw no people on the way up OR on the way down. That in itself was pretty stinkin’ cool. The first couple hundred yards of the trail are muddy (like, for reals), so be prepared. There may even be walking sticks at the trailhead (which is here).
Total round-trip distance for the most direct route to the top: 8½-9 miles.
There weren't any waterfalls, per se, but lots of forest (the trail does not intersect any of the logging areas). The hike is difficult in spots because of downed trees, but with the storms last winter, I think trees will be a common obstacle on trails this year.