Unlike Tac I wouldn't say the language and platform are irrelevant. Learning anything is a time investment and you don't want to invest time into something with very limited value going forward.

You need to figure out what kinds of projects you eventually want to be working on, then choose your miniature "self education" project accordingly.

If you want to create something other people might actually use while picking up employable skills then I might suggest doing a Java/Android or ObjectiveC/iPhone smart phone app. There are books available that walk you through the process.

I would also point out that much of what makes someone a good developer has nothing to do with knowing a particular language or SDK. Object-oriented methodology and design, data structures, algorithmic analysis, memory management (esp. in garbage collected languages where it tends to get ignored), etc.