M.M.
Just in case you didn't already know, there is the Hyde Park Parent Support Network (http://www.hydeparkpsn.org/) that has a group listserv where lots of different local and city-wide activities are posted. PSN also has a playroom where different age playgroups get together (posted on the website). Someone already mentioned the Hyde Park Neighborhood Club that has a tot lot (paid) during the week. If you are at all affiliated with the U of C, there is the Family Resource Center (http://grad-affairs.uchicago.edu/services/parents.shtml) which has playrooms and different playgroups meet up as well as planned activities, some free while some for a small fee. There's another group that runs a Friday morning tot lot, calling it Winter Park, for free but is on break at the moment. I believe they are starting back up after the holidays and are located in the basement of St. Paul the Redeemer Church. There's also the winter session of Marsha's Music (http://www.marshasmusic.com/) starting up next week (on the pricier side but I believe worth it for the energy burning aspect and social time for mommy).
I hope those resources help. And congrats on your newest member!
mom to 15 month and 4 month olds