Photo credit: Atari
Before engineering a lunar lander in class, I try to provide early finishers an opportunity to try some of the following lunar lander games. This is a good way for them to recognize that they really need to slow down their crafts in order to keep them from crashing, due to the force of gravity. The first game to try is Lunar Lander from Atari.
Also try:
The Lunar Lander simulator from PhET. I believe that this one needs to be downloaded, and may use Flash.
This Lunar Lander game is from KidsAstronomy.com
Moonlander G-8 also uses Flash and is from Moonlander.eu