Yeah one of my biggest regrets was not making checkpoints/goals clearer, especially since some require multiple bodies to hit them while others only need one. If i work on it in the future I'll definitely add a sort of start-of-level description like poly bridge has.
I hadn't considered alternatives to grid/non grid position/velocity snapping but it would be super cool if once you placed a body the velocity could snap into place (maybe by holding control or some other button) to the nearest velocity that establishes a stable orbit. And then maybe an alternative grid that is radial rather than rectangular with the origin at the current center of mass of the system, or a given selected body or bodies.
And since you seemed interested in the implementation, I used a method i found described here: https://en.wikipedia.org/wiki/Numerical_model_of_the_Solar_System#Integration, where it seems better than a standard leapfrog integration, but not as expensive as Runge-Kutta. I also calculate all the bodies' updated positions first and then update them all after so that symmetrical setups don't break as quickly.