Update: Well, my trekking was short lived. The van came around a few minutes late, then we stopped to pick up a few more people. When everybody had clambered in, Ben, our guide, said the trip would be "we drive to three waterfalls, have lunch, drive to mountain, drive to karen village, do something else, back to chang mai". I thought the trip was "drive to park, hike up to mountain, hang out on mountain a bit, go down to village or elephant or something, then back". So, I said it wasn't the trip I thought it was going to be and got out. Then I walked back (1/2 hour, so much fun).