Take the ferry to St George.
Then take the bus to Crystal Caves.
After touring the cave, walk to Swizzle Inn for rum swizzles and conch fritters.
Take the bus to the aquarium.
After that, take the bus to Hamilton.
Either walk around in Hamilton and then take the ferry back, or just take the ferry back.
It's a long day, but you're doing everything ala carte so the cost is very reasonable.