Believe it or not people, it is possible to have fun and still be "on-brand." Why are we so serious about advertising anyway? Or rather, dear client, why are you so serious about your brand? Let's just have some fun with it!
Case in point - see the video below. I love this. Look at all the people that stop being lazy and start taking the stairs.
Think about using FUN in your brand messaging. Or I don't know, making the buying process or purchasing environment FUN. Like these guys:

Hotel ZaZa "Houston" Guest Room

IPod Nano

Anthropologie Store