My most ambitious mission was the Grand Tour: I visited all landable bodies in one trip.
I also did a huge space station once, it was the jovian gateway (totally not inspired from the LOPG
): a space station that was in orbit around Jupiter, placed in a quasi-satellite configuration around Europa. It was meant to permanently house an astronaut team, and to act as a local control center for future manned missions on the galilean moons.
The project was huge, my patience not as much, I didn't finish it