Baseline Tennis Center at the U of Minnesota offers group drills Thursdays and Saturdays for $25 a session. Although it's a pretty mixed bag of folks who show, from roughly 2.5 - 4.0. The advantage is you get an hour and a half of court time indoors for $25, which is a steal compared to everything else around here. Plus if there is nobody on the courts afterward you can stick around and hit some more.
I'm intrigued by the thought of indoor courts free of membership and charge though!