I am looking at the Disney excursions and none appeal to me. I wish there was a low cost, beach day with a turtle farm trip thrown in. Maybe a local restaurant with traditional ethnic food of the area for lunch, and a short walking tour. I see nothing like that, so I am considering piecing it together myself.
First, I really recommend checking out the Ports forum on cruisecritic's message board. They have so much info.
Second, from that forum I have read that there are trustworthy guides for hire, under canopies, once you get off the ship and across the street from the port area. You could see if anyone would be willing to do that for you and be your driver/guide.
Third, the beaches are all right there (assuming you want one of the beaches along 7-miles beach). Just look at some tourist sites for GC and pick one. We are doing a RCCL cruise that includes GC, and I think I've chosen Tiki Beach as the one we will go to. We'll pay a little bit for chairs (the named beaches along 7-mile are all public and free, but if you want to use a chair/lounger/umbrella/etc then you pay the people who own those) and have a nice beach time.
I don't want to visit the Turtle Farm, but if I did I would just get a cab. I have read that they are plentiful.
Food...again, just look at maps of the area. Look on the ports forum. Find out what IS local food, even.
I personally am intrigued by a Chinese/Indian place, but it's all the way on the other side of the island, and that makes me too nervous (even though I'm sure it would take very little time). But I'm easy with "local"...if it's made by people who live there, I'm good with that.
Find a place and catch a cab.
Walking tour...I haven't heard anything negative about GC, so I figure you could just, again, look at some tourist guides/sites, choose what you want to see, and create a tour for yourself.
Just be sure to look at the Navigator for what time the ship is on, make sure you have a proper watch (not something that's going to change time on its own, like a cell phone that forgets it's on airplane mode, LOL) set to the ship's time, and make sure you're back WELL in advance of all-aboard time.