One of my new projects this year is a game design unit in which students program their own Frogger-type Game. We use a software program called AgentSheets, which allows students to program in a "drag and drop" programming type environment, providing an easy introduction to programming without the need to teach 7th graders a programming language. You can use the software to create any type of game. You can check out some of the games my latest group created here.
I learned of this software when I taught in Boulder. The Boulder Valley School District was participating in a University of Colorado study lead by Alex Repenning, the creator of AgentSheets. I had the unique opportunity to be paid to attend a week-long class during the summer, gain graduate credits, and use the software for free with my class in exchange for participating in this study.
I have found that my students love using this software and creating their own games. The interest level is extremely high, and it's great to be able to give them the opportunity to get a taste of programming without having to spend weeks learning a programming language. This software can also be used to create science simulations, so it can easily be integrated into other curriculum units. One caveat is that although it's fairly easy to use, there is a steep learning curve, as it's likely to be very different from anything your students have done before. Be prepared to go slow. If you can get a helper or a student to be your "expert assistant", it will help a lot!
If your budget won't allow you to purchase this software for your class, there are similar free game design applications available online. One of these is YoYo Games. Although I have not personally tried this software, my coworkers have told me that it is great as well!