I've been playing flightgear, an open source flight sim. It's loads of fun to just fool around with

I've also tried Rigs of Rods, which is a simulator for all sorts of vehicles. I haven't spent too much time with that because it doesn't have built-in joystick support So I need to get a program for linux where I can keymap joystick buttons

With both of the games, I think it would be really fun if you could select a character/pilot, and then use the multiplayer options for a virtual hangout-type thing