Animate several objects simultaneously
Give them different speeds and directions
Stop or bounce back when they hit edge of panel
YOUR_PANEL.getSize().width
, ...height
Give them different appearances
General background: http://www.java2s.com/Tutorial/Java/0240__Swing/Timerbasedanimation.htm
Primitive example (but don't use sleep() like they do): http://codereview.stackexchange.com/questions/29630/simple-java-animation-with-swing