Do something really cool and fun with buttons and sliders. Add sound if you like. For extra credit try to implement buttons that have all the correct functionality, but have wacky shapes or rendering styles. Or, if you're feeling ambitious, try making a two dimensional slider, that produces two different values (one for horizontal position, the other for vertical position). See if you can use widgets to control something animated. Or you can try to use them to make a game.
Creativity and daring will be rewarded. :-)