I have a standard project that I do whenever I learn a new language. The reason I do the same project each time is that I already know all of the business logic, so all I need to focus on for the new project is the language and syntax, not the business logic.

My chosen project is a MUD, which is a text-based MMORPG, effectively. I already know how the TCP connections work, how to load areas/zones, how to establish characters and monsters, and how all of those things interact. So all I really need to learn is the language.

So my advice would be to pick something you've already built in the past in a language that you're vary familiar with, then port it to the new language. It's easier than coming up with something from scratch.

Your mileage may vary, but this generally works very well for me.