The Li River runs from Guilin to Yangshuo. Some people take a cruise between these two points but we don't recommend it, for, not only is it expensive, it's, frankly, something of a bore. The most spectacular part of the river is the section that flows from Yangdi to Xingping. Small motorboats owned by farmers make the trip between these two points. They are unlicensed, so to catch one, you need to arrive very early in the morning or in the late afternoon, as the police conduct inspections the rest of the day and it's difficult to find a boat willing to take you. In any case, these are the best times to be on the river, as its calmer, the big boats that come from Guilin are absent, and overall experience is more authentic.
To get to Yangdi, it's best to take a bus from the station (60-90 min, 8CNY). Once in Yangdi, go to the wharves and ask (most likely, you'll be bombarded with offers). As they aren't official, they usually are slightly removed from the main docks.
Make the trip to Xingping in motorboat (3 hr, 100CNY). The views are breathtaking. Look out also for the interesting local way of fishing. When you get there, very often the boatman doesn't drop you in the same town (to avoid security controls). Make sure at least that you are let off on the correct side of the river!!! (taking into account the direction in which the boat is going, it is in the left bank). In Xingping, you can enjoy the market and later take the bus back to Yangshuo (30-45 min, 6CNY).