Best way to tour with limited time?

[CaribQween]

We will be in Grand Cayman in February 2010 on the Carnival Legend. We have a diving excursion booked for the morning which is supposed to return us to the pier around 12:00. Our ship departs at 4:00. So, we would like to do some touring around during this time. We realize that we would have to be back on ship by about 3:30 and since this is a tender port we have to leave plenty of time to tender back to the ship. So we figure we have about 2 1/2-3 hours for some sightseeing and shopping. What would be the best way to see a few of the highlights such as Hell, 7-mile beach, etc.? Would we be able to just grab a cab and ask them to give us a small tour or could we take a bus around to the different areas? Any suggestions would be greatly appreciated. Thanks in advance.