Ok when I say addictive I mean it, be very careful. If you've got work to do leave this thread now. http://www.bridgebuilder-game.com/ I'm no way affiliated, it's been around forever. It's quiet simple, you have a 2d "level" with a stretch of water, you need to construct a bridge to get a train across. Of course the train is heavy, so you often fail miserabally and send a few hundred people to their deaths. You also have a budget, each "length" of bridge costs money. Classic, simple, easy, fun and ADDICTIVE.
its fun, trial and error mostly, physics are pretty debatable though. too easy to mess up though cuz of the mouse control