You could create a bouncy ball where the mouse can move it up and down, and when they let go, it will bounce off the floor, decreasing in height every time it bounces. I would maybe just do it going up and down first, and if you think you are up to the challenge of adding the side to side option, you can try that after you have the up and down done. Keep it 2d though.
The challenge would be having the ball go increasingly faster the further it is above the ground. Maybe you could research some gravitational calculation functions off the internet and use it for the speed per second of the bouncy ball.
This will require the use of physics and gravity, so if you don't think that you are up to it, then you can do something easier, but it would be a nice challenge if you could get this done and working realistically. I might even try it if I get interested enough.
Check out my best projects so far and please comment:
Tic-Tac-Toe 2.0 ................... http://www.alice.org/community/showthread.php?t=3548
Alice Paint ........................... http://www.alice.org/community/showthread.php?p=23408
Real-Time Clock .................. http://www.alice.org/community/showthread.php?p=12725
Maze Game ......................... http://www.alice.org/community/showthread.php?t=4301