Any simple game usually does the trick: snake, Yahtzee, black jack. The rules (requirements) are well defined. The pieces (UI) are well defined. That way you can focus on actually implementing the logic in the language/framework of choice. For websites, pick your favorite restaurant and make it. Borrow the text, pictures, etc. and just focus on the code that creates it and not get off on a rabbit trail doing images in paint.net or making up items.