I made a little game where the mouse moves a block and the other blocks are the problem. Some are good, some are bad, and the idea is to keep moving rather than run into the wrong one.
It is a Java applet using AWT drawing and a mouse-motion listener. The player and moving obstacles are block objects; the game keeps them in an ArrayList and checks the player against them. It also has score and time state, so there is something to watch besides the blocks.
Like Pong, it uses an offscreen image for double-buffered drawing. The rules are small enough that I can experiment with the game loop and collision checks without needing a lot of artwork first. Apparently a square and a few moving objects are enough to keep me occupied.