For us, development is like a personal journey - never ending strive to innovate and to create new levels of quality - it was, it is and it will always be our ultimate motivation.
Prototyping, brainstorming and re-engineering is a bread and butter of creative development process. Also, it's highly addictive - has anyone heard about term "code junkie" ?
Stuff. Algorithmic stuff, complex stuff and shiny stuff. Anything that will reduce your costs or make your clients' jaw drop.