On the front of my cards, I usually put the theatre's address, phone number, box office number, fax number, SM numbers, CM number, and rehearsal hotline. On the back I put the performance schedule (though this can be a bit tricky if it's a long run).
In addition to wallet cards for the cast, I like to do an SM only wallet card that has the phone numbers of the entire cast and crew on it. It's much handier than carrying the contact sheet around with me at all times.