When you are at the port there are folks holding up signs for $30 tours that will take you to the Turtle Farm, as well as other stops.
When we were in the Caymans last year we took a bus to 7 mile beach on our own. We went to Royal Palms Beach club, it was me, my wife, 5 year old daughter and 3 year old son. It was $2 a person to get into the beach club, the kids were free since they were 5 and 3. Then you rent a beach chair on the beach, that was $15 I think. There is also a pool, bathrooms, bar and food you can order as well. We are going to stop there again when we are back on the island.
The buses are just long vans, they don't look different then the taxis which are long vans as well. We got into a bus to go to Royal Palms, you tell them where you want to get dropped off. It was $2.50 a person for the bus and $4 for the taxi each way.
On our upcoming trip this year I think we might do the $30 Cayman Island bus tour. We are thinking of going with Captain Marvin's to schedule a private tour since there will be 10 of us.
Cheers!