What RobS said.
Hartford is way too small for that length of trip with not enough to recommend (I've done a day trip there for the Mark Twain house). Yes it does have a commuter and Amtrak service to NYC.
Indianapolis has a zoo a raceway (the Indie 500 is in May)

Cleveland could be fun if there is baseball on the go - it's not the "mistake on the lake" that people call it. There's the rock and roll hall of fame, a nice zoo etc. but I'd struggle to make 5 days out of it but it could be a nice quirky (different!) place to visit.

Philly has a lot going on, great public transport, and you can skip up to NYC or down to DC if you need to. Of the four, that would be my choice 100%.

One last thought....you could do Boston and take the Amtrak Noreaster and do some time up in New Hampshire and Maine. Leave it until mid October and you could get some great autumn scenery.
Good luck with the trip!

PS Others may have an opinion about what routes will open up more Avios opportunities, depending on the class you are looking to book....but destinations with more flights each day may be better for that? To that end, any of EWR/PHI/BDL/BOS can be a start/end point with decent links to others. E.g. EWR has a direct train service to Philly.