I can think of 3 basic ways to handle a roleplaying session.
1) Star Trek Style: The PCs are in a location that Has Some Problem. They deal with the problem because they're in the location. The problem is the star.
2) Dungeons Must Kill You: The PCs are traveling through a location that Tries To Kill Them. They put up with it because they want something that's there. The location is the star.
3) The Matt Method: The PCs want something. I help them get it. I hinder them from getting it. The PCs and their goals are the star.
I like #3 best. But you can't use the same bag of tricks every time, unless you want bored players. So for the big Zal Duster throwdown -- a classic #3, selected by the players at the previous session as The Thing We Should Do Next -- I put in some Dungeons Must Kill Them for variety. Any time you can include a menagerie of menacing moon-monsters, you should.
The PCs, of course, never visited the menagerie.
Also, 30 seconds after we started playing, they said "What if we don't try to take Zal Duster down after all? What if we just go do something else?" Mischevious grins.
I'm not perturbed. "I'm prepared for that. I've got a backup encounter ready to go." Not totally true -- it was a mishmash of vague ideas rolled up in a #1 -- but it calmed them down. All they had to do was figure out how to actually go after Zal Duster, which I'll pick up next time.