MikeG is looking for suggestions for ONLY one day. We ran with one, offer them more.
If I had suggested mainland, I don't quite know what they would want to do. I would agree to the mainland as well. What does your group like to do, MikeG ?
Even Ac and CC are not all about beaches and bars. Many shop owners would tell you that.
Boat rides are always nice, just go on a pleasant ride to the mainland with the water taxi. If you don't want to do anything specific there, turn around and ride back. Many people take pleasure in nice ride as well