Currently playing the most engrossing and nerdy game ever:
Kerbal Space Program.
On one level, it's really very simple: you get to design, build and fly rockets and spaceplanes, blast them into space and run a space program. So far, so good.
On another level, if you want to get any of the really cool stuff done - like staging, docking, orbital construction, landing on planets, etc - you'll need to get your head around orbital mechanics. Thrust to weight ratios, orbital maneuvers, phase angles and loads of other mathematical stuff are needed in order to take advantage of the Newtonian principles of getting from A to B in elliptical motions.
...or you can download a mod and use the autopilot! :D
It's currently in alpha... but it's a really, really good alpha with lots of development and an active mod scene. The trial is free, but its the 0.13 version of the game. They're currently on 0.18.2 which has a lot of advanced features.
Definitely worth a look if spaceflight interests you.