For great fun, go around the neighborhood trying the doors and windows of people who appear to be away from home. Once you get in, leave a sarcastic note on the kitchen counter.
That’ll teach 'em to be more careful about locking up. 
Print this (pdf) to it.
Or maybe this slightly longer (pdf) version.
“I’m alive but trapped in the Wi-Fi.”
“We are sending you a message from the future…”
“…###//… $%###NSA##$$$… @$%%%%%…”
“Don’t forget to drink your Ovaltine.”
Type out “I’M A PRINTER” in large, bold type, and set it to print 100 copies.
“Pass the word along. Tell the men it’s time to Shoot the Moon!”
Hi, I’m from the local Assembly of Jehovah’s Witnesses. We would like to know if you are happy with your religion and with this printer. We have been thinking about buying one like it to print out The Watchtower.
If you want to be a good neighbor, print the page of the instruction manual which describes how to set a wifi password.
Please visit[size=6]
http://boards.straightdope.com/sdmb/showthread.php?p=19556640
kthxbye[/size]
Not only does one of my neighbors have an unsecured printer, she also has an unsecured wifi. A wifi named her street address. Which is so naive it is scary.
Lol. I like this one
I think you should go for the long game. Today a one pager that just says HI. Tomorrow, a “Hi, just checking in” Next day, “Why are you not responding? Is something wrong?” Etc, etc. Could go on for weeks.
To make it extra creepy, you could add some detail about the neighborhood. Like “Hey. what’s with the guy with the red shutters leaving his garbage can out for days? Trash pickup is on Wednesday, pal.”
“I can see you”
“I know what you are doing”
“You should be ashamed of yourself”
“I’m going to the police”
etc…
Print a dating site bio:
Single black printer ISO stainless steel toaster / white microwave / black modem. I like printing pictures of flowers & picnic baskets, etc.
This is getting better.
How come you can’t access the network thru the printer?
Moderator Action
(Note - this isn’t my forum to moderate but I’m guest modding here by request due to the technical issues involved)
The laws are admittedly a bit murky on this subject, and there are 50 different states and 50 different sets of laws which vary greatly. Unauthorized access of wireless networks is generally prohibited in most states, with some treating it as a criminal matter and others specifying monetary damages only.
Either way, accessing someone else’s network also often violates the terms of service of their Internet Service Provider, which usually has a clause against sharing the connection with neighbors or anyone else who is not in the residence.
A lot of these laws and regulations are admittedly vague (many don’t actually define what exactly “unauthorized access” is, for example), but here on the SDMB we prefer to err on the side of caution. This thread therefore falls afoul of our rule prohibiting illegal activities.
This thread is therefore closed.