BIG WARNING!!! you will need serious bandwidth, but my school is lucky in this sense. I screencasted the idea below, but you basically go to appear.in and set up an account and then create rooms for each of your students, like appear.in/room1, appear.in/room2, appear.in/room3, you should have more creativity than me. Then you post the links via symbaloo, shared google doc, phone conversation, whatever, with another class. It works just like a todaysmeet.com but with video/audio and ability to share screens which is the cool part.
You could do a million things. Modified #mysteryskype, collaboratively work on a project, writing, ask each other questions, debate, etc. The ideas are endless.