I hear what you're saying about time. But, frankly, you can find a handful of cooperative activities that are as easily implemented as anything else (actually, I find cooperative learning sessions much less difficult than activities centered upon me.)
But, first, it involves developing group work habits. I know this might seem a bit out of character for me, but often I just give grades on how the kids do three or four little things like (particularly early in the semester):
1) sit close to each other 2) lean forward, don't lounge 3) say nice things to each other
And actually you might have a few free minutes to sit down and do a little work while the students are involved in their lesson.
Actually, I wrote a little article on producing some small boards for cooperative learning purposes. These boards are really great. Easy and cheap to make. If anyone's interested, I can e-mail.