Couple months ago I started working on space shooter/simulator, just for fun. Newtonian physics, html canvas. It had 2D top view (F2 key) and 3D view (F3 key), but because canvas is quite slow on fullscreen I usualy switched rendering off (F1 key) and then I spent most of the debuging time there. I only needed few data to be displayed so I only rendered speed, distance fuel etc. as numbers. Then I added yaw pitch roll indicator and even though it was on canvas, because only small part was changing it was kinda fast, so I started adding more and more avionics and eventually the instrument flight [1] aspect of it become much more interesting that realistic planet renderings and I completely neglected 3D view [2]. The most fun so far was programing docking autopilot. So if you haven't make your own space simulator, you should, it's fun. (unfortunately it's not finished and not even in publishable state)
[1] https://drive.google.com/file/d/0B5SCh06PKpVtZHowN1l0TEw3cGs...
[2] https://drive.google.com/file/d/0B5SCh06PKpVtUzMxSGNhRy1vVlE...