Interplanetary commerce is a great theme for a campaign. Some planets will have a surfeit of certain resources, while other planets are bound to lack sufficient quantities of the same thing for their purposes. Entrepreneurial characters can make a killing carting these valuable commodities around the galaxy, from planets where they are dirt cheap to planets where the natives will pay through the nose for them.
The interesting thing is, these commodities don't necessarily need to be the sorts of things we conventionally think of as valuable. Gold is kind of scattered around anywhere there are rocky planets, so you could mine it locally and not have to buy imported stuff. But water...
Actually, bad example. Water is pretty darn common in the universe. If it isn't on a planet, then it'll almost certainly be found on asteroids or comets or other planets in the same solar system. There's absolutely no need to cart it across interstellar distances or invade other planets for it. Certain science fiction movies notwithstanding.
In fact almost any raw material is going to be had relatively nearby, at least within the same solar system, more easily than bringing it from another star system. Which means the most valuable interstellar trade commodities will probably be biological, technological, or cultural.
Yep, the vast star-spanning civilisations of the distant future will not be trading gold , silver, or jewels. They'll be trading ice sculptures, fine wines, and music videos. Remember that if you run a campaign featuring interstellar commerce.
Han: Chewie, how's the ship coming along?
Chewbacca: Progressing roughly at pace with your ability to achieve anything useful here on this snowball, old chap.
Han: So that's good, then?
Chewbacca: Let us say that it is no worse than anything else that is happening to, oh, for example, any of our friends at this moment.
Chewbacca: And another crate of duct tape would be useful.
Han: Hey, I just had another great idea.
Chewbacca: Allow me to pre-empt your brilliant plan: We are not carving up a glacier into exquisite ice sculptures to sell to the Hutts on Tatooine.
Han: Ooh! That's even better!