I agree with the others--it will be a crowded weekend.
In the summertime--I would stick to the beach or the swimming pool. The parks are too hot, the water parks are too crowded, Sea World, though lower stress--still hot and outdoors for most of the shows.
If you need an activity:
DTD has fountains (shooting water type from the ground) to have fun in.
There are the museums on I-Drive--Ripley's, and that Wonderworks place is supposed to be cool.
http://www.wonderworksonline.com/
There is mini-golf next to Radisson at the Port--you can go to jetty park and walk along the pier to wave to the cruise ships leaving on Thursday.
You can come down to Melbourne (where I am--30 minute to 1 hour drive from the port, depending where you stay)--
We have Andretti Thrill park
http://www.andrettithrillpark.com/
It has race cars, arcade, laser tag, et cetera.
Brevard Zoo is a nice little zoo down here. Not very big--but we have giraffes, so we are getting somewhere finally on the zoo scale
Hope this helps--enjoy your cruise!