Invest in a decent joystick if you're going to go the flight sim route. I would recommend the X-52 pro as a good starter.
I can concur that the DCS series, particularly A-10C, is pretty amazing. Just learning how to start the engines from a cold start takes a lot of time, but if you take the time to get to know all the systems, its hugely rewarding. Also a big fan of IL2, old game but great WW2 flight sim.
On the car front, invest in a decent wheel and pedal set. I have the Thrustmaster T300 and friggin love it.
iRacing is amazing for competitive racing. Like real racing it can hurt the wallet, but the competitive multiplayer experience is second to none imo. I'm still pissed with myself for buying Project Cars.
If you want to experience the open roads of a micro europe while listening to your favourite tunes or streaming radio stations (you can add local stations which bridges a little reality to the game), euro truck simulator 2 is a very immersive and relaxing experience.
Either route, would highly recommend getting a head tracker. I cant sim without one (at least until the vive comes out).
http://www.naturalpoint.com/trackir/products/trackir5/