Apart from presenting an interesting problem, I'd say presenting an opportunity to learn something new and valuable is as important. I developed and maintained

Apart from presenting an interesting problem, I'd say presenting an opportunity to learn something new and valuable is as important. I developed and maintained an ASP.NET application for a long time and eventually became bored. My boss tried to make things interesting by giving me small new application / feature to solve a problem but having to continue using ASP.NET made my gnash my teeth. I would have preferring having to figure out some new language/platform where the discovery process would have been rewarding and satisfying. So give them not only new ends to pursue, but also new means.