We did an excursion of the harbor and also Atlantis...its really something to see, they give you a tour and you get to see the dig, huge aquarium and if you are lucky to be there when they feed the fish, its so cool! At the end of the tour, they gave you a ticket to take the water taxi back to the pier. If you go alone, you'll need to take a regular taxi or the water taxi...we thought it was worth seeing.
Atlantis is amazing! You don't need to book a tour to get there. Either grab a water or land taxi over to Paradise Island. The casino has some breathtaking sculputures, you will have to pay to tour the aquarium/ dig. worth it!