After being in Chiang Mai for a while, you may hear foreigners talk about a waterfall you can climb up like a gecko climbs up a wall, and they might refer to it as the "Sticky Falls", or maybe, the "Limestone Waterfall". You might hear a Thai person talk about "Buatong" or "Nam Phu Chet Si", and casually refer to it as a nice place to visit. They would all be talking about the same place, and anyone will tell you it is an attraction well-worth experiencing, probably unlike any other you've had before.
What makes this waterfall different to the hundreds of others in Northern Thailand, is that its fed by a calcium-rich spring at the very top, which rushes downwards over the boulders of the waterfall. This makes them feel and appear quite "sticky", and unless you've had much experience groping calcium-hungry rocks, you will be shocked at your ability to grip them with your bare skin. On top of that, they look rather cute, being all smooth and rotund, and almost seem like fat little steps when you climb higher up the waterfall.
The falls are about 50km north of Chiang Mai, in a very quiet, tourist-free area, sans the occasional Thai family or couple. There are places to stay nearby if you want to build a weekend around the falls, such as the Mae Ngat Dam Houseboats, or the Rabeang Pasak Tree Houses. It's also a good place for a day trip, and as the area is so isolated and undeveloped, it might be wise to bring a picnic and a few beers. You can't take any food or drink down to the actual waterfall, but there are picnic tables at the top level, and plenty of wild forest to explore.