 A few suggestions
Class surveys are a good way to get larger groups to circulate. Give the students some time (10-15 minutes, or as homework) to formulate their own questionaires related to whatever you're teaching in class. Then they all have to get up and interview X number of people in the class and record their answers. Afterwards you can talk as a class about the questions they made and some of the answers they received. This gives a little bit of structure to students who are intimidated by "free-talking", and relieves the pressure of speaking in front a large class.
Another one I did with a very large class of military officers (!!) was provide a few discussion topics on the board, such as "how did you meet your wife" and "what would you like to do when you retire" and have them work through them in pairs. You could do this at the start of each class to get them warmed up.
HTH
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