Some places I'm familiar with -
Hand of Man at his best:
1) Bok Tower Gardens in Lake Wales - beautiful setting on highest point in Florida peninsula
2) Leu Botanical Gardens in near Winter Park - formal & informal gardens
3) Frank Lloyd Wright buildings at Florida Southern College in Lakeland
Nature:
1) Blue Spring State Park, Orange City
2) De Leon Springs State Park, Deleon
Also check out Clyde Butcher's Living Waters-Aqautic Preserves of Florida that includes nearby natural places to visit.