Answer by jfar for Becoming a multi language programmer
Original Question/Answer on StackOverflow.com
Write something fun.
For example I’m in the opposite situation ( well, I’m no C# guru ) and getting back into php. For a first project I’m building an application to store the different types of fireballs stored in street fighter games. 
Keeps you motivated and you can kind of explore the programming space a little more than you would if you implemented a boring blog system or shopping cart.