If I were going to set up a no-password guest account on my machine, I wouldn't call it "guest." Any script-kiddie on the planet will look for "guest." So how about, say, "gus?" Or "frisbee?" Or "pinochle?"
Make this an ordinary-joe user. The only things accessible to him are public things (which are read-only to him), stuff in his own private folder, and temporary files he made. Use the quota system to make sure he can't create unreasonably-large files. If you use ssh to provide access to your system from outside, put ol' gus into the list of users who cannot log-in that way.