As for everything, you have to pay to get to the Moon Hill. For 15RMB you get into the "park" and can leave your bike (should you cycle here) at a guarded lot. You can also get here by bus for about 3RMB, then you have to flag a mini bus at the street to get back to Yangshuo. If you can't find a bus to take you directly to the entrance, take one to the ticket office of the Water Cave and then continue on food for about 500m on the same road. taht said, you can combine a visit to the Water Cave with the Moon Hill: when coming from the cave you will get droped of at the ticket office, early in the afternoon. Then just walk to the hill.
There is only one real way up the hill but there are some extra loops, taking in some diferent perspectives of the Moon Hill and the surrounding. All pathes are very slippery when wet. Also the track is relatively steep. Still you can make your way up in a sweaty 20min or so. The first time you get out of the forest you'll find yourself just unter the arch. There is a platform ofering some good but not special views of the surrounding. Well, there is the view through the arch. That is something you won't see too often elsewhere. To get its view walk along the path through he arch to a second smaller platform. You can take a path that brunches left of this way to get to the top of the arch. This muddy path however is officially restricted.
I spent a little more than an hour at the Moon Hill, getting up and down and shooting photos.