All right, I'm creating a calendar. It's slow-going. So far, I have added some astronomy happenings, events at the Bell Museum, Tamarack Nature Center, Festivals and Fairs happening around the Twin Cities (hey, I know these are not necessarily nature-y, but they are outside and usually free and there's probably a way for you to get some science and nature stuff out of the experience), and a few other things.
At this point, I am organizing it by region or sponsor (in the case of the University of Minnesota). I have a whole bunch of nature centers to add, as well as parks and don't even get me started on the fantastic stuff happening in our Minnesota State Parks.
If you have a suggestion for things to add to the calendar, please let me know in the comments!