Princeton Health Care System has a lovely group called Bright Beginnings. They meet Wednesday mid-morning at a gym near 206/518 intersection.
http://www.princetonhcs.org/Default.aspx?p=360&d=11706
The website is a bit of a pain, but the group is great. Meet other new moms and learn a little something too.
I have also enjoyed their mommy & baby exercise class on Alexander Rd and their Postpartum Adjustment Support Group.
Also check out www.princetonkids.org they have a weekly email The various libraries in Mercer county and the Princeton Library are all nice and do stories at different days and times, mostly for 6mo and up, but if you just need to get out...
Oh and the MarketFair mall usually has many moms walking strollers to stay in the cool (or out of the snow!)
Have fun and best wishes!