At the bottom of Sonic receipts there is a phone number to call to get a free Route 44 drink after doing a survey. When you get your free drink from the free coupon, you will get another receipt with another survey for a free drink. So basically you can just keep getting free drinks.
These are fountain drinks so you can’t really “stock” up on them but if you are driving around and get thirsty it would be a way to get free drinks.
If you like slushie type drinks, check out this cool machine to make your own at home…
Nostalgia Electrics Retro Series Slush Machine - only $36.26 (NOTE: Amazon prices do change without notice)