A fun game I like to play is “hunt for the chupacabra.” In Google Docs and Google Presentation, when people link to your document, but are not implicitly shared* with your document, they join your document as an anonymous animal.
Create a Google Doc or Presentation.
Make the document publicly viewable or anyone with the link can view. You can also set it to anyone with the link can EDIT. I like the can edit option when I share with students so I do not have to worry about them remembering their password in order to participate.
Share the link with students (or out on twitter). I like to use http://goo.gl to share a shortened link.
Tell the students we are on the hunt for the chupacabra.
CHEER when the chupacabra shows up. It is a fun community building activity to have a goal and reach it. The students will enjoy working on collaborative documents when they can be on the hunt for the chupacabra!
*I have noticed sometimes someone NOT implicitly shared will show up as themselves and not anonymous. I can not yet figure out why. I know if you insert a comment you stop being anonymous.