Another one is simply setting up an access point broadcasting "Free Internet" then watch who jumps on.

[AP] [Silent Proxy sniffing traffic] {Internet}

And technically, they connected to your network so you may have some legal argument against wire-tap charges for that silent sniffer in the middle. I still wouldn't recommend it without an initiall "welcome to Free Internet, all your traffic will be monitored while here" or similar method of assumed permission. Still, it's a possible attack criminals can easily make use of.