Shelby, you will have a great time on the Fantasy, she was re-furbished about a year ago and is in great condition. They have an awesome water slide at the pool that you and your children will enjoy. The Camp Carnival program is a great one also so your kids will have a blast.
As far as excursions, my only advice would be to choose maybe one or two, otherwise you will wear yourself out and not be able to enjoy everything on the ship. The Blue Lagoon is a great excursion. You have a 45 minute boat ride thorugh Nassau Harbor to Blue Lagoon. They have non motorized water sports for rent such as paddleboards, tubes etc. You can lay on the beach and soak up the sun, they also offer snorkling. They offer a full lunch menu and beverages. Then there is a 45 minute boat ride back to the ship. This excursion lasts about 5 hours. It is one of the most popular of Carnival's excursion. Of course if you want an excursion that has a little more party atmosphere you can always do the Yellow Bird Party Boat. It is free flowing rum punch (non alcholic for kids) a private island beach and snorkling offered for a fee. That one lasts about 3 hours, but believe me, the partying is the best part, there is a calypso band on board to keep you entertained.
Both of these excursions are usually in early afternoon, so you would still have time to get off the ship in the morning and explore Nassau, the Pirates Museum, and shop. Just a word of caution, while in Nassau you will be harrassed by some of the street vendors. They will even walk up to you and tell you that they are giving you a free bracelet or necklace, try to put it on you then after you accept it they will ask for a donation. Just tell them NO and walk away, don't even let them try to give it to you because if you accept it they will follow you and keep asking for money. Around the straw market they are really forceful, but just say a firm NO, not interested, and eventually they will leave you alone.
By the time you finsih shoppping and doing one of the excursions, you will be ready to go back to the ship, enjoy it and get ready for dinner.
Hope this helps some.