Wow, what a great trip. I'm jealous.
A buddy and I did something similar to that at 11 or 12 years ago. We went to Cooperstown, drove across to Boston for a Bosox game, and back home to NYC to catch the last game of a Mets series and the first game of a Yanks series.
If you get to Yankee Stadium early, they opened a Yankee museum at the Stadium but the line is long, so be prepared. It is like a mini-Hall of Fame, literally. Also, there's always Monument Park in Centerfield.
Like I mentioned above, when you go to CitiField, grab a Shake Shack burger. One of the best.
When you go to Boston, please do me a favor and remind the Red Sox fans that Boston sucks!
Philly fans--the language coming out of the mouths of the children down there (or up there in your case) would make a hooker blush.
Enjoy and safe travels!