Using whatever is set-up....email list, SMS list, Facebook friends,Twitter followers.....whatever....
Just use passwords.
Example: this Tuesday the password is: Joe sent me........and they get half price on a cheesesteak
Send a message that customers who stop in on Tuesday and say "Joe sent me" at the register will get half off the price of a cheesesteak.
Don't need any coupon website or mobile site to promote specific deals.