"Cool bananas" is an expression of appreciation, endorsement or an exclamation of enthusiasm. Similar to "cool beans" and awesome!
Etymology: Possibly originating in Australia in the early 1990s, loosely based on a television commercial for an Australian automotive reseller whose tagline was "Cool Banana Motorama"
Statement: I just got us front row tickets to the show.
Reply: Cool Bananas!
Statement: You can now play for free.
Reply: Cool Bananas!