Membership to Longwood Gardens (if they like gardens) or Winterthur (if they like history, nature walks, or Stately Homes as the Brits call them). They can go to various events, exhibitions, and so forth, or just go for a lovely walk, any time during the year. Personally I prefer Winterthur. I believe they have reciprocal membership with several Philly gardens, but double check that as there are usually distance restrictions.

Alternately, a membership to the museum or garden or nature center of their choice, keeping in mind that they are more likely to go to places nearby their home. Consider also Hagley, anything near you that you could go to together, the art museum, and so on.