I did most of the islands last year when we stayed on raillay. Phranang Beach is worth spending a half day or full day on with all the other day trippers we were fortubnate as we stayed here as our base it is stunning.
Long tail boats generally you will be quoted the same kind of price from them all. We had one the first day and just booked him for all the other days. The offshore islands are wonderfull for swimming and snorkelling we paid 1500 baht for the hire of the boat for the whole day to the islands offshore which are chicken island, poda island, and my favourite tup island, we did this trip 3 times. Tup island has a sand spit between it and the fish will eat out of your hands in the shallow water.The boat man will stay with the boat all day and look after your belongings everything will be quite safe we left anything we did not want to leave on the beach on the boat. Some of these islands will be deserted when you get there anyway. If you like snorkelling these guys know all the best stops. Our boat man did not want to leave us he just sat on his boat all day and stayed with it under his brolly felt quite sorry for him.
He also took us to the hong islands which are beautifull with some nice beaches this is much further and he charged us 2500 baht it takes about 1.5 hours sailing to get there. You can see a pic of one of the offshore islands under my hotel review of the rayavadee.
I would not recomend you doing phi phi in a longtail it is too far but it is worth going by speedboat.
Ao Nang is very well developed and you should have no trouble with ATM's there. Loads of restaurants and shops.
I did not worry about any of my belongings on the beach but we only carry enough money for our day out just in case. I did not hear of any pick pockets. If you go for a swim just make sure anything of value is not on show.If you require any more info just ask.