[Haskell-beginners] Training tasks

Hi. I'm building website where people can try and "taste" new languages by solving small or mediums size tasks. Tasks are language specific and should show best features of the language. Website is not meant to teach new language but to give idea what is this language good for. Now I want to add Haskell. I need about 7-10 tasks for now. First three of four tasks are introductory, they should show/check basics of haskell. E.g. given n, return sum of squares of first n even numbers. Other tasks are more complicated and show advantages of functional programming in general or some specific haskell features. I don't have any experience with haskell and I need you help. Could you help me with ideas for tasks? Thank you, Nikita Beloglazov