Great Parents Great Starts offers some wonderful playgroups; some are for babies birth-1 yr, some are for babies 1-3 yrs... etc.
There's a May calendar here:
http://www.resa.net/downloads/early_childhood/calendar.pdf
We're hooked into a playgroup in downriver that meets in the evening, and really enjoy the ongoing contact with other parents dealing with the same developmental issues that we have!
Also, there's a ton of free outdoor concerts during summer. I love bringing my children to these and picnicking outdoors; it exposes them to music, and the outdoors, and other families.