If you're just looking to develop software, pick X language / framework of the month (i.e. node.js for web, C# for Windows applications (this may be expanding with Mono for OS X / linux), C for low-level control, etc). If you're just looking to learn about the diversity of languages, there's a number of styles that you can play around with. Learn about type systems and compare how you would design an interface in strict vs. dynamic vs. duck-typed systems. Try out manual memory management in C/C and learn why garbage collecting is so much less of a headache. Try writing programs with completely deterministic run-time. Compare syntax between python and lisp. Investigate how even the simplest programs are translated into assembly. Try Scala / Racket for alternative ways to interface with Java frameworks. Your question is pretty open-ended, so it's hard to provide more than a laundry list of different tools and applications...