Text a giant sign
Here's a business idea for both mobile phone companies and people who operate those giant digital signs in public places (such as malls and the Times Square jumbotron.)
Let people text a message to the sign for a lucrative but affordable fee. It would then display ASAP, though possibly a human would have to check for "offensive" messages, whatever that means. You could see people putting up love notes to their valentines as they both go by the sign, rivals having battles and debates in their messages etc. Could be both entertaining and lucrative. The texted billboards (or from a web form with graphics) would contain a bar with the texting number or URL to enter your own. If it were cheap enough you might see crowds stopping to enjoy the battles on the jumbotron.