I'll give a thumbs up to Rock of Bral as well - there are a lot of ideas to steal from it, and all of them won't show up in the setting book. And the main boxed set has a lot of good stuff in it - including lots of pretty ship pictures - that can be of use and is good for inspirational material.
I think a lot of the Spelljammer accessories are good for inspirational material, but be aware that a lot of it suffers from a Star Wars/Star Trek problem of trying to summarize entire worlds in a few pages. With that in mind I like all of the various *space books (Greyspace, Krynnspace and Realmspace - probably in that order), Practical Planetology and (to a lesser degree) Crystal Spheres. All of them have ideas for planets/systems that you can rip off when you need a planet for an adventure. (Don't get me wrong - I think they all also have problems, but if you're mining them for ideas they all have their uses.)
I'm less likely to recommend the Astromundi Cluster than a lot of other folks - it always felt to me that it failed at one of the key conceits of the Spelljammer setting, in that the premise is that it's a closed off sphere and once you're in it you can't get back out. If you ignore that and just use it for its pieces I still don't personally like the pieces as much as the other supplements above - it's a more cohesive setting, so the pieces are harder to break apart and use independently IMO. And while it's an individual taste thing, I don't particularly like the setting as presented and wouldn't run a game in it (It also has an origin for Mind Flayers that IMO is not great and I'm glad that everyone has decided to pretend it doesn't exist).
Also I'm going to put in a plug for a non-Spelljammer D&D book that I think would be useful to look at if you can get your hands on it - The Plane Above from 4e. I've been re-reading it recently for an epic-level side game I'm running for a couple of my players and there's a lot of good stuff in there about the Astral Sea that might be inspirational too (and lots more pretty pictures of ships to use as well).