In Universe Sandbox you basically have all planets, stars and galaxies " in the palms of your hands " so to speak. You can create your own worlds or play with existing presets. I like that you can change the values of each object, for example making the sun too hot which results in the planets basically disappearing.

A downside is the lack of visual effects. You can change the mass of a planet, but it still looks the same. When things collide there are zero particles, so if you look away for a second you won't even notice that a collision happened.