Again... I don't know where in PA you where thinking... but there are some small estate homes in Center City Philadelphia that have outdoor gardens, however they will only cater to smaller parties

Physick House

Powel House

Morris House Hotel (I love these pics)

Kimmel Center of the Preforming Arts (If you have a large party the roof is awesome)

Colonial Dames

If you are looking for something unique I would try the Fuel House, National Constitution Center, the Franklin Institute.

http://web.mac.com/jlyaron/FUELhouse/Welcome.html

Also go to Party Space to check out PA sites:

http://partyspace.com/philadelphia_weddings/reception_sites/by_location/default.asp?st=Philadelphia%20County

great idea really cute!

that pic is so beautiful!  i love barn weddings.

Here is some more info on Colonial Dames... there was someone who had a really cute self-uniting ceremony there, but I can't find the pics. BTW, if you didn't know you can preform a self-uniting ceremony in PA