Reducing the fare is the first thing that I would look at. I have no brand loyalty, so price and port are my number one and two considerations. Next would be the on board credit. I like to pick out my actual cabin, so upgrades don't really thrill me.
For me, I would also be excited if there were other things like dropping or greatly reducing the charge for a third or fourth person in the cabin and most importantly, the single supplement charges. As a single, I would like to travel without paying for myself as if I were two people.