A great way to spend your spare time in Yangshuo is to kayak along the picturesque Li River (Lijiang River). You won't find any white-water but there is no more a tranquil activity than a paddle along the river in rural China, especially on those hot summer days!
For more of an adventure on the Lijiang River, you can arrange kayaking trips through China Fact Tours. Trips typically last 3 or 4 hours including equipment and transfers to and from the river. The routes are generally easy, traveling downstream, though during the rainy season the currents are stronger.