There's always Principia for your n-body needs [0].

[0]: https://github.com/mockingbirdnest/Principia

Seconded. Principia is amazing; first thing I did after installing it was set up a Kerbin-Mun L4 orbit.

Another thing I found, due to the full n-body integration Principia adds, is that I was able to correctly guess which side of Kerbin the Mun was on due to tiny perturbations in my craft's equatorial orbit.

Knowing that this exists might be what convinces me to finally get a reasonably up to date computer.

Then play Orbiter (http://orbit.medphys.ucl.ac.uk/), which is to KSP like building a house is to playing with lego.

Orbiter is underrated compared to KSP, which is recommended nearly always when the topic of orbital mechanics comes up. Orbiter is not only free(ware) but also a fairly realistic space flight simulator.

Perhaps the realism/implicit difficulty is the reason it's less recommended?

KSP is enough to give you an intuitive understanding without the gritty details of which switches in which order.

That said I did just pop up a video of orbiter to check what I remember... And the oceans are solid.. at least KSP has some model of water landings?

Is it as hilariously fun as KSP though?

Windows only. D':