What I really like to do, more than flying and landing (which are fun), is learning how to cold start these planes. Firing up the engines is really satisfying.
However, not all the planes have detailed cockpits and some of the switches aren't mapped out to do anything. I'm guessing a good flight sim would let you have access to more systems than this free game does. I'm not knocking flightgear, because it's actually great. I'm just wondering if there's something you use and recommend.
Some screenshots I took from flightgear:
