For all of those who love cosmic catastrophes, here's a simulator for solar systems.

[DetlefK]

http://www.stefanom.org/spc/#
This was invented by an astronomer.

It's a game. The goal is to add as many planets as possible and as heavy as possible to your solar system WITHOUT something going wrong.
But watching things go wrong is funnier.

[Igel]

1. Sort of cool.

Provides the occasional optical illusion. Helps demonstrate what resonance would look like.

But there's no gravity between planets.

https://phet.colorado.edu/sims/my-solar-system/my-solar-system_en.html isn't as nice looking, no real presets and is worse in some ways, but at least includes gravitational attraction between the different bodies. My biggest complaints are not having distances in AU and not allowing the mass of the star to be changed.

(Sorry, used the phet.colorado.edu thing in class.)

[DetlefK]

2. What do you mean??? It does have have gravity between planets.

If you insert a dwarf-star, the center of mass moves so far that you can see the sun wobble.

[JayhawkSD]

3. Quite cool.

Kind of fun to add a planet for the specific purpose of making another one crash.

[DetlefK]

4. I think, I found a "winter is coming"-scenario.

Earth-like planet in a certain orbit. At a slightly larger orbit and at APPROXIMATELY the opposite side a gas-giant.

The center of mass is then to the side of the sun. When the Earth catches up to the gas-giant from behind, the gas-giant's gravity pulls Earth into an orbit between the gas-giant and the sun.
But in this lower orbit, the gravity of the gas-giant then pulls Earth slightly outwards, which makes it go into a higher orbit.

Earth oscillates back and forth between orbits close to the sun and orbits far away from the sun.

This means, some years would be exceptionally warm and some would be exceptionally cold. In the simulation, a warm-cold-warm cycle takes about 50 years...