We did visit Vero Beach Resort when our girls were teens, and there was plenty for my DD's to do.
Of course, the pool and beach there offer a great opportunity for swimming, sunbathing, etc. Also, there are wave runners etc. that folks enjoy renting, though we never did.
There is a small mini golf course at the resort, as well as gift shop, community hall, etc.
We typically do short stays there, so I am not sure if you are staying longer than say 4 days that they may get bored by then. There is a mall and cinema not too far, but you definitely need to drive to get to it, perhaps 10 or 15 miles I am guessing. Good luck in your planning..