Yesterday I posted an item on how fun can change behavior. In response one of my contact at BT plc passed on to me this link to the playpump system (a charity sponsored by BT).
This is how a simple insight (fun can change behavior) can be turned into a life saving solution.
here is a 30 second clip on the system
if you want more info this is a 4 minutes insight from national geographic
There is nothing that states that your work should be boring to be rewarding. There is no rule book that holds that nothing of value can be achieved without pain.
As it seems easier to buy the idea that customers should have fun rather than employees, the place to start investigating the value of fun may be with your customer experience. How can you make your product/service something fun for customers to interact with (and yes the more difficult and complex it is the more value there is in making it fun).
Once you've done that and realize the value of fun there will be nothing stopping you bringing it back inside.