Having done some game design myself in the past, I imagine gravity works as some version of 9.8m/s/s; how hard would it be to replace this with a PLANET_GRAVITY variable and have slightly different gravity on different planets? I think that some variety between 7 and 14m/s/s would be fun and give planets different feels. Since exoplanets are smaller, they could have even lower gravity, sort of like playing the borderlands presequel (“moon” level gravity). Planets could also have different colors of rain, snow and fog to fit their planet palettes.
Just a few ideas to further give different planets slightly different feels.