Since it's 42 & I am fond of the number..

The 'most fun' I had was making a completely useless feature for our customers that we (our team) knew will be useless (&wrong) once finished and we will have to rewrite it. But we had to do it nevertheless till the end of the week, since the customer is the king. It turned out hilarious and fun because everyone was making jokes on the floor about what idiotic stuff we code and implement. Even the boss was like: yes, yes, I know but please do it, you can rewrite it later to not do anything, just leave the button on gui. It was crazy it was fun, a little bit of mindless coding to lighten up the atmosphere and it (coding & jokes) brought closer the whole colective reaponsible for that particular customer.