How to make them interesting/unique/fun?
A single drop of water in a desert is interesting and unique, but quite mundane in a tropcial region.
The concept of interest/unique is similar to the concept of contrast. To create something unique and interesting you need to create something mundane first.
Well, what are mundane planets ? Are there terra-formed earthlike planets, populated planets, desert like planets, alien planets, water planets etc. ?
A very hi-contrast will make planets like bonfires. Eg. if most of your planets are desert like, with small colonies or even only a space station, then a tropical planet inhabited by aliens will be like a bacon the player want to explore. On the other hand, if most planets are terra-formed, containing some kind of wildlife, then a alien populated planet has not such a strong attraction and uniqueness.