"A cool drink to hang out with" is just a catchy slogan for an Appy fizz commercial. It's basically just an anthropomorphism, to say the drink is like a (cool) person you would want to hang out with; probably coined because people also say "a cool person." P.S. "To hang out (with)" = "To spend time with, socially." Usually only used by teenagers and below. Adults don't really 'hang'.

To add to Mark's answer, there's a double-meaning played for "cool": temperature-cool, and friendly-cool. Rather obvious but I thought I'd point it out anyway... Appy Fizz is the only companion you need! :)

Simply means, it is fun to go out with