Elite:Dangerous is the newest installment of the same series as the C64 game, and is a sandboxy-sort of online multiplayer thing.
Star Citizen is another (in dev; currently you can buy into the public alpha-sort-of thing) one, produced by Chris Roberts of Wing Commander fame. It's being sold as a "first-person universe", because you can also leave the cockpit of the space ship and run around places, optionally shooting things in an FPS sort of manner.
Of the two E:D can be played on slightly lower-end PCs, but SC is pretty demanding of a computer, especially in its current mostly-unoptimised alpha state.
Both of the games are pretty fun, but have their problems too.
E:D can be a lot of fun in small doses once you get your feet wet, but can be pretty grindy to start out with and gets kind of shallow and repetitive later. Also it's super-amazing to play in VR, if your computer can handle it.
SC is looking hugely promising and the current alpha gameplay is a lot of fun if you can deal with the bugs, but has a lot internet drama surrounding it. Said drama is mostly related to the game taking forever to develop, and the fact that you can currently pay (sometimes rather insane sums of) real money for getting your hands on the space ships currently available. The devs are insisting the buy-ships-for-money thing is only happening during development to help fund the process and will no longer be possible after proper launch, but we'll see how that works out in practise.
[Hellion](https://www.playhellion.com/) looks promising as well - it should be out for prerelease soon. I expect that to take up my time while I wait for SC.
If you want small planes or airliners then you might look into the new X-Plane 11, it has very realistic physics and looks really good even in the basic configuration, i.e. without add-ons, thanks to its last-generation graphics engine.
If, on the other hand, you plan on buying many add-ons (e.g. because you want to fly specific planes or fly to/from specific airports) then I suggest Prepar3d or FSX Steam Edition which are both based on Microsoft Flight Simulator X, which means they are compatible with a large number of add-ons.
Their graphics are not on par with X-Plane 11 though (unless you install scenery add-ons), since they are based on a 10-year-old graphics engine; you can see a comparison here: https://www.youtube.com/watch?v=ZUtEqgDjjJU
But BOY does it look good! And the stock 737/MD82/747 are approaching payware-quality.
KSP, in its unmodded state, isn't very demanding GPU-wise, so as long as your CPU isn't 20 years old, it should run fine.
In fact, it runs acceptable on low-medium settings on my 2014MBP with integrated Intel Iris(!) graphics.
Civilian aviation: X-Plane, Flight Simulator X Steam Edition, Prepar3d. With IVAO or VATSIM if you want multiplayer with real humans doing air traffic control.
Space: Orbiter or Kerbal Space Program for simulation. Elite for combat.
It's arguable how much flying you do in this, but radio navigation, communication, planning and aircraft management are pretty interesting on their own.
I fly only small, one or two prop planes and gliders, and do so pretty much by the seat of the pants -- no GPS, even an altimiter is not really that critical. I mostly just look out the window and use an airspeed indicator to make sure I'm not coming in too fast or slow.
I was just landing Piper Cubs this way all day today. Now steering those things on the ground is a real pain, even with real rudder pedals. My taxing skills are pretty weak.