$15 for a 1/2 hour round trip to the island from the Kep Pier, about 5 minutes from the Crab Market. ($35 for the whole boat which can hold 10 people). You can do a day trip, or stay overnight.
There's a half dozen beachside bungalows on the island from $5 to $15 if you want to stay here. Otherwise, be sure to get the boat back in the evening, as many times all the bungalows are filled up.
It's a 3 hour walk to go around the island. You'll be on beaches, mangrove forests and rocks.
You can bring your own food, or stop by one of the half dozen restaurant on the beach.