Noppharat Thara beach is about a 15/20 minute walk from Ao Nang. It's a long stretch of beach and is less developed. The northern end past the canal and ferry pier is even more secluded area where at low tide it is possible to cross to a selection of 'islands'.
The pier at Noppharat Thara is where you'll need to go to pick up the Ao Nang Princess to Phi Phi Don.