This post will focus on a couple of cool tools geared towards helping kids explore different physics concepts.
The Fantastic Contraption is a web-based tool that presents a variety of different puzzles for students to solve by building different contraptions. Each puzzle essentially consists of having to move a given object across a given distance. You can work through the 21 initial levels without even signing in or creating an account. Upgrading to the "full" account for $10 gets you access to other levels and puzzles created by other users. It also allows you to create your own levels. For most teachers, access to the free levels will be adequate.
Physics Phun is similar to the Fantastic Contraption but it is a desktop application that needs to be downloaded. Versions are available for Windows, Mac and Linux. Phun has quite a few more possibilities than the Fantastic Contraption because it allows you to create your own puzzles from scratch and the possibilities are nearly endless. You can create spacecraft, vehicles, bikes and all sorts of other forms of transportation and then test them out over different terrains and in different situations. There are a variety of videos showing Phun in use and some of them are included below. These videos will help give you an idea of the kinds of things you can do with this software.
A basic tutorial (a little slow but a good overview).
A tutorial showing how to create a rocket (I only included this because of the interesting musical accompaniment).
A sample video showing that you can even create a mountain bike and test it out.