This website is not designed for learning, but there are interesting things to read, practice and to find out about current affairs in Serbian educational system.

One of the articles that I find interesting is about numbers: http://zelenaucionica.com/pravopis-pisanje-brojeva

There is also "Igramo se i učimo" section (Play and learn) where you can find interesting tasks and even stories to read: http://zelenaucionica.com/igramo-se-i-ucimo/priche

Which website do you find useful in learning Serbian?