There's no shortage of boatmen on Ko Phi Phi and they'll all happily offer you a range of boat trips around the island.
Several join-in snorkelling half-day or full day trips are available, too.
The highlights are Monkey Beach (the real one, not the sliver of sand south of Ton Sai) and Lo Moo Dee. These are somewhat inconveniently located at opposite ends of the island, but that just gives you a better excuse to go all the way around the island.
While it's easy enough to get a packed lunch before you go, most of the beaches on Ko Phi Phi have at least one guesthouse or resort so it wouldn't be difficult to plan a meal stop half way through.
Trips are generally priced by the hour and the distance involved. Expect to pay in the region of 1,500 baht for a half-day, 3,000 baht for a full day for a trip involving a few stops around Ko Phi Phi. There is scope for bargaining though and these prices are for the boat, not per person.
If you can manage to wake up early, The Adventure Club runs three-hour "shark watch" trips starting at 06:00 off Long Beach, where you'll have the chance to snorkel with the reef sharks.
There are other options available, sunset cruises and booze cruises on yachts to name but two. The hot ticket on our most recent visit seemed to be bioluminescent plankton snorkelling tours, which run in the late afternoon to evening. Any one of the 42 zillion travel agents on Ko Phi Phi Don will be able to assist.
The Travelfish newsletter is sent out every Monday and is jammed full of free advice for travel in Southeast Asia. You can see past issues here.