There is plenty to do in the area and you are not that far from some of the major theme parks if you and them are into that. We did this type of trip about 10 years ago when my girls were 13 and 10 and we would go somewhere one day (Disney, Universal, etc.) then spend the next day just laying on the beach and swimming. Make sure that you take your girls out on the beach after dark (take a flashlight) and the little crabs come out and you can make the crabs "chase" the girls. You will have a great time, enjoy.
Go to the Daytona Beach website, there is plenty of ideas there.