The past couple of weeks I've been working on a prototype of multi-planet gameplay for my lunar lander game. Results are not terribly encouraging, so far.
One of the big problems is figuring out how to miniaturize the solar system without getting totally wacky gravity. On Earth's surface (if I've done my calculations right) the acceleration due to the sun is about 1600 times weaker than the acceleration due to the Earth itself. Thus it doesn't much matter where the sun is in the sky; up is always pretty much the same direction.
I would like to have a smaller, denser solar system to cut down on travel times between planets. Unfortunately in my experiments so far this results in slanted gravity as the sun and other nearby bodies move across the sky.
Our solar system has about 99.9% of its mass in the sun. Thus, there's very little gravitational influence from one planet to another. As I distribute more of the solar system's mass to the planets this makes it harder to fly between them due to the perturbations. (It would also make their orbits unstable but I'm restricting them to only pay attention to the sun's gravity.)
I'm also hitting numerical precision issues as I expand the play area, so I'm trying to track those down.
I may end up falling back to having an “overworld map” for travel between planets, similar to Gravitar. This would lose some of the wow factor that continuous flight between planets would have, but you could operate on different time scales in the two modes and reduce boredom and precision problems.