If you ever go back, here's a few other things you might want to look up.
There's the former Henson townhouse up at 117 E. 69th Street. They've taken out the stained-glass windows so it might not look terribly obvious. Just look for a building with dark-colored bricks and white columns - if memory serves it's right off the Lexington Line.
Yes, there is a caricature of Kermit in Sardi's - we took my grandmother there for her birthday once. It's on the second floor, near the restrooms and the entrance to the second-floor lounge.
There's also St. John the Divine, where they held Jim's memorial (and later, Richard's), on Amsterdam and 110th. But for Muppet fans, what's really worth a visit is the Paley Center for Media on 52nd Street. They've got a huge archive full of Muppet movie and television clips, from the big three shows to all the previously unreleased pilots and pitches and stuff. Good times.