Another common visual object is a candle flame. Stare it at for about as long as you can stand it, then close your eyes. Stare at the after-image on the back of your eyelids. For me, it solidifies into a red dot within about thirty seconds of steady concentration or so, and if I can keep it up after that it starts morphing into a 3-D shape. It's very cool.
The part about this method that I struggle with-- keeping the eyes closed. My physical eyes immediately try to "look" when something interesting happens and it blows the whole thing. It can be quite frustrating.