Great Gardens that are worth a visit:
Phipps Conservatory (Pittsburgh)
Longwood Gardens (Kennett Square)
Chanticleer (Wayne)
Welkinweir (Pottstown)
Jenkins Arboretum (Devon)
Tyler Arboretum (Media)
The last five are all outside of Philly–you could def. make it to each one if you plan your route. My personal favorite is Welkinweir.