Having fresh eyes learn the product and then focus on user experience yielded some great insights. From there, most of the time, we slowly moved into user support. Afterwards, I'd assign special projects depending on personality and interests. One had a really amazing knack for front end design, so she ended up rebuilding most of our support site. Another was just a brilliant, compassionate writer. She rewrote most of our tutorials and translated many of our support materials into French. A few months in and she had advanced enough to become my replacement's most important resource. I still can't quite believe her growth over a short time.
In retrospect, I don't think the particular tasks matter as much as the mindset. I gave them important tasks that they could handle and, as they learned more, I gave them more. And, I know that our entire company trusted them. Finally, this only worked because upper management completely supported me.
The one that stuck out the most was downgrading their desktops from Vista to XP. Most of the drivers on the XP disc didn't work on the newer hardware so i was pretty stuck. One of the engineers told me to research slipstreaming drivers, which led me on a 3 day journey to creating custom boot discs & successfully installing XP
Give them work that will challenge them & also serve a business purpose if possible