Build anything from jets to spaceships and do whatever the hell you want with them - fly around, crash, orbit the planet, plant a flag on the mun or fly into the freaking sun! Or you could build a complicated system of satellites, refuel your ship on one of them, recharge at the other, redirect a meteor, then slingshot on minimal fuel to a far away planet and back (bonus points if you manage to take a surface sample)! p.s. as a nice bonus there are some great mods (free, not corrupted by valve & co.) for this game.
Great old-school game with great writing, a lot of choice and all the great stuff you'd expect from a real rpg.
