I'm working on implementing an integrator with an adaptive time step, so that the time step is high when masses are far away from each other and just coasting,

I'm working on implementing an integrator with an adaptive time step, so that the time step is high when masses are far away from each other and just coasting, but low when they get close to each other and interesting things happen; think the orbit of a comet getting altered as it encounters Jupiter after a long journey from the Oort cloud. As things are now, such a scenario would be painfully slow to simulate if you want to maintain accuracy, but if you speed up the time step, nothing remotely realistic will come out of the simulator... So good point, and I hope I'll be able to cobble together a solution soon enough :D. As for symplectic integrators, I'm not sure they stay symplectic if you alter the time step, but I could be wrong!