[bennywild]

Pick something you want to make and make it. The best guideline for this task is to scope small. For example, try and code a clone of Breakout. Now try and do it in a language you've never used. Maybe try and render it server-side and send the info to a canvas element on a web page.

The other direction you could go is implementing algorithms in various languages as a sort of mental workout, but that's a lot less fun.

[pawn]

I'll second this. My "Hello World" is Pong. Whenever I want to test how well I know a new language, I make pong in it. It's relatively simple but will allow you to play with several fun things like animation and button mapping. Plus, you can show it to almost anyone and they'll look beyond its simplicity and think about the potential it represents - that you can make almost anything else if you wanted to.