Tonight while doing a little bit of research and spending some quality time with my son we had planned to visit a site we found out about the other day. Armadillo Run located at http://www.armadillorun.com/ is a fantastic game that pits players skills around building structures to help an Armadillo to get to a certain location in space. It plays much better than it sounds.
There is a demo to download (only 1Mb in size) that takes you through a really simple and easy tutorial which provides you with everything you need to know. My son enjoyed reading the instructions and then guiding me through the process. The game has realistic physics properties in play that makes it highly addictive.
I liked the game as it was very simple with only a small range of building options, but incorporates budget considerations and allows players to apply tension, timing and a few other settings. It even allows you to create your own challenges for others - my son particularly liked this one as we could build challenges for each other to test our thinking and construction skills. We do need to buy the full version but it is worth it simply for the conversations and fun we had dealing with construction, scientific process and exploring physics concepts - all while have a laugh.
Check out the video Fun Motion - then download the demo and have a go - sorry PC only.