If you're a baseball fan, or at least know the history of it all, a trip to Wrigley Field is incredible. It will probably be a bit chilly in April, so plan accordingly. Pick up some cheap clothes in town if need be, and drop off at a charity before you leave (unless you have the luggage space.)
The inside has changed since it opened way back when, but overall it's still pretty similar outside. It's kind of overwhelming (again, if you know/appreciate the history of it all) to be standing on the same corner where the goat incident was, for example.
As for your overnight options, me personally I'd go in order of ATL, BOS, pause a good while and then consider JFK/LGA. I would not even consider EWR.