Definitely Chanticleer in Wayne, PA; also Winterthur, Longwood, Nemours. Contact the PA Horticultural Society via their website and they will supply you with many good places to visit. They have a booklet. It is a rich, rich area. Chanticleer is absolutely stunning and has an intriguing "ruin garden". Nemours has a "runi garden" as well.

Well, I don't know if it falls under the category of "gardening tours," but the Philadelphia flower show is one very good gardening reason to visit that city!

Bartram's Garden in West Philadelphia has great historical interest. Might want to read The Natures of John and William Bartram by Thomas Slaughter or Travels by Wm. Bartram. Have fun. Philadelphia is wonderful and has great gardens.

Pam,
How was your trip if already taken? I would like to hear about your experiences if you have some time. Here's a helpful link that you hopefully found already.
deb Lux

Here is a link that might be useful: http://www.libertynet.org/gardens/