There's gravity from the sun but not the planet, and yes, that ought to be worth adding. I'm not sure what'd help the most to turn this into a fun game, though, so I'm working on other things. Thanks for the suggestion; a time control's probably a good idea too.
Ah yes some kind of normalization for the sail's velocity would be cool. The whole systems dynamics gets a lot slower as you get away from the sun, so it gets frustrating to see what happens with those; and it is hard to execute manuevers near the sun. (Maybe normalize for constant angular speed and display e.g. 'Time at 3.5x'?)