We visited Phuket at high season, so it was difficult to arrange long trips out to Koh Phi Phi and we were also warned it would be very crowded there in any event, so we opted to stay close with the exception of a short day trip to some nearby islands where we snorkled and fed bread to the local fish. A feeding frenzy erupted and it wasn't just the bread the fish sampled. Ouch!
Getting around Phuket is expensive unless you rent motorbikes which rent for about 250 Baht ($7.00) per day. Just make sure you take photos of any scratches/body damage to the bikes when you take them out to avoid an extortion when you return them.
Finally, Kata Beach and the warm waters of the Andaman Sea are so nice that we didn't really want to wander far in any case