I hate to say this...but that time of year where indoor playspaces are paramount to having a good day is quickly approaching.  Mass Premiere Sports in Foxboro, MA is a great place for kids under six to exhaust themselves.  The space is a huge indoor basketball court littered with all kinds of toys; ride-ons, playhouses, balls, a roller coaster (Step 2), and a moon bounce.  Admission is reasonable and the staff is very nice.  The open play area is closed for the summer but they are set to open that area back up in early September.  Very close to routes 95 and 495 http://mpcourts.com/info.php?page=location.  Mass Premiere Courts also offers birthday parties.  My daughter and I attended one and it was great!