With respect to the battleship sudoku by motris I will give here an example of a skyscraper sudoku (how knows my page could know it). This variant wasn't invented by me but I think it is still interesting to solve it:
Fill the grid with the digits 1 to 9. The digits represent the height of the skyscraper in each cell. Each row, column and 3x3-box will have exactly one of each digit. The clues along the egdes tell you how many skyscrapers you can see from that vantage point.