EteRNA is game that's becoming very popular amongst my circle of friends.
Once creating an account, you are able to play the "game" in which you must design RNA sequences that fold up into a target shape on your computer. The beauty of this game is that you learn about RNA as well as eventually contributing to the project's purpose, which is to create the first large-scale library of synthetic RNA designs.
I found it easy at the start, but it soon became quite tricky once I moved onto the challenges. Be sure to check it out. Have fun, learn, and contribute!