For a limited time, you can get a FREE Medium Slush at Sonic Drive-In — no purchase required! Simply download the free Sonic Drive-In app for Android or iOS then create a new account and verify your email. Once you've done that, your free slush offer should be available right away. To redeem: visit a participating Sonic Drive-In, open the app, click on "Redeem" then show the coupon to your carhop.