Wagner Free Institute of Science. It's a natural history museum that was founded in the Victorian era, and which has retained its original design and layout of specimens for over 100 years. It's like stepping back into time, and yet also absolutely timeless and relevant.
The Wagner has education programs during the school year for classes that want a field trip. I actually like it better than taking kids to the Academy of Natural Sciences because it's just full of specimens and has very little text around. At the Academy the experience is very directed and there's a lot of information to read. There's something wonderful about just having access to their incredible collection with minimal direction- it fosters curiosity and exploration.