Phi Phi is definitely a sun, sand and sea destination. There isn't much to do that doesn't involve being on the water, or under it. The sheer rock walls surrounding the islands look like they might be good for climbing, but I didn't see any outfits offering that sport, like there are around Railay.
The main village on Phi Phi Don, as seen from the viewpoint. Tonsai Bay is on the left, Loh Dalum Bay on the right.
What you will find, though, are lots of dive shops, even though the scuba diving immediately around Phi Phi is not that great. It must be that there are so many people passing through Phi Phi that they can capture a few of them. Apparently, the introductory "Discover Scuba" offers are their stock and trade. See the Phi Phi Islands dive guide at SEAundersea.com for more information about diving around Phi Phi.
On top of the water, there are popular day trips to the more remote parts of the island, as well as to other islands such as Phi Phi Le and Mosquito Island. If you get tired of the water, you can take a walk up to one or more of the viewpoints on top of the line of hills making up the eastern side of Phi Phi Don. The walk up from Loh Dalum Bay to the first viewpoint takes about 10 to 15 minutes. There's a small shop at the viewpoint where you can buy drinks, snacks and souvenirs. A second viewpoint (pictured above) is about five minutes' walk further up, and it has another, larger shop.
Worth noting: The "grab all you can" attitude of many boat tour operators, coupled with an almost complete lack of government supervision, means that boats are sometimes dangerously over-crowded. Keep in mind that you, and only you, are responsible for your safety. If a boat seems unsafe, get off of it.