I always learned new languages & frameworks by solving a pretty simple but not too simple problem: a recipe manager with persistence and the ability to organize recipes in a calendar and generate a weekly shopping list, sorting the items by supermarket aisle and in the last release I even did a task manager integration to share it with my wife (I think it was with Todoist). I did it with C#, Python+GTK, PHP and Rails.

Now I'm using Paprika and the need is forever solved, but as an exercise in terms of managing general UI, database persistence and relationships, and calendar & dates is hard enough without being overwhelming.

Other people create binary file editors, this is my problem of choice ;)