Hi zenn
The islands are mostly uninhabited..so there's no accomodation, no vendors, no nothing. If you choose to go to just one island, I suggest you bring a good book, towels, lotsa sunscreen, food and water and snorkelling gear (they sell them at Ao Nang beach). Of coz if you take the island tours, food & drinks are provided, as well as the snorkelling gear. Sunscreen is a must..you can easily get a sunburn out there, even when you're in the water. For the tour, I think we stopped at each spot for around 1/2 hour to an hour. Yes, staying on one island for the whole day can be boring. We did that when we were in Phuket...a full day trip to just 1 island. I was so bored, and was getting roasted..was wishing we cld go back but the boat wld only bring us back at a fixed time. However, if it's not a tour (ie you don't need to adhere to their schedule), I think you can just arrg with the long tail boat driver on when you want to go back. Since you've nvr been island hopping before, perhaps it's best you do the 4-island one to get a taste of it..then if you like, hire a long tail and go out on your own the next day.
Railay beach is on the mainland, but you need to take a boat to reach there. There's no road access from Ao Nang coz it's blocked off by a limestone cliff. It's a favourite place for rock climbing enthusiasts. Poda is an island that's visible from Ao Nang beach. At Ao Nang beach, you will see the long tail boat rates..they'll advertise how much you need to pay for each destination.
For a better idea of islands, beaches etc...google Krabi destination guides...I used to bookmark some good ones, but I've seen deleted them from my faves..else I cld send you a few links for reference. But it should be easy for you to find the info on the net.