You are right that Manuel Antonio will be packed at that time. We would suggest trying to find a smaller beach town or something more secluded. One place that comes to mind for great wildlife and beaches is Drake Bay. Drake Bay is a small and remote village with not that many hotels so won’t be as busy. We just looked and the hotels there do have some availability left for late March. Check out our post Drake Bay: Costa Rica Unplugged for much more information.
Answer for Avoiding busy festival weeks 2