If you're looking for something that allows you a lot of freedom to set up and play with simulations, you might want to consider an open-source C++ project I've been working on for the last few years as part of my PhD. Most of the graduate students in my group use it now, so there's a fair bit of man hours that's gone into it.

Bit of a shameless plug I know ...

The toolbox is called Tudat (TU Delft Astrodynamics Toolbox) and you can get it here.

Shameless plug for Tudat (TU Delft Astrodynamics Toolbox)...

If you're looking for something that allows you a lot of freedom to set up and play with simulations, you might want to consider an open-source C++ project I've been working on for the last few years as part of my PhD. Most of the graduate students in my group use it, so a lot of effort has gone into it.