I'm personally not a fan of including planets in ACtA. It just throws things out of proportion for me. Yes, I know there are scenarios and stuff that include them, but I just don't care much for them. Planets were always back drop elements, because they are FREAKING huge. Engagements didn't take place around a planet, they took place to the side of one. Also, using a planet a couple inches across puts movement per round of some ships at near FTL speeds. Probably my biggest issue. (Using a 5 inch planet earth model and assuming a round is somewhere on the scale of 30 seconds long... how long is a round actually, if stated? this puts the white star traveling an average of around 1400km/s... if I did my math right...)
Anyway, as for terrain, your most common object to use is going to be asteroids. When it comes to the junk in space, those are your most likely terrain feature. Gas clouds are another, the predominant item for those are cotton balls. I've been reminiscing about
Homeworldrecently, and was thinking about making some MASSIVE alien relics. Ever since I read 2001 a million years ago (the part where , I love the idea of derelict alien structures that are just massive (like a piece of a
Dyson Sphere). Things that would make Babylon 5 look like a toy model. Such creations would dominate the background of some Homeworld levels and helped to give the game breathtaking backdrops.
If you go through the Gallery thread, there are some amazingly well done asteroids and other back drop material posted along side some of the ships