I would say a good goal/challenge rather would be to see if you can re-write a system at work by thinking outside the box and use a technology you would like to improve on. For example, if you would like to play around with Node.js and at work, you use C# and a strongly relational database, try re-writing it in Node.js along with different technologies, like using NoSql for example or a Redis Cache, even bring in Docker containers into the mix, just to spice things up. You will be amazed at the things you pick up, and some of those things you will able to apply at work. I found that doing this gives you insight into the systems at work as well as other solutions no one at work has thought about, so basically, a win-win.
Also, constrain yourself to the problem the system you are re-writing solves. By having this and this as the only constraint it makes your brain think of really creative ways of solving/approaching the problem differently and gives you a chance to use what you have learned in the new stack very creatively.