Usually what I do to get the creative juices flowing, is to imagine my world as Disneyland. Every location should be unique and interesting, as should every character. Its kind of like going sight seeing. If there's nothing to be seen, then why should your players care? So I often start with a location that I'm interested in. If for example the players are hired by a wizard, then where does he live, and what is special about it?
No town is a generic fantasy town, and no wizards tower is a generic wizards tower. I want to be invested, so that my players are invested too. So this is where I would start. Where does this wizard live, and what does it look like? What is the wizard's motivation for the quest, and do his interests perhaps reflect in the look of his abode? Note that the home of a wizard in itself is a fantastic location where all sorts of magical stuff can happen, before the players even get to the quest. Have them freely explore in such a location, and you'll easily fill a whole session without ever getting to any battles or the quest... and that's fine.
But once the players get going, try to imagine what would be an exciting location to have battles in. What sort of environment embodies the feeling you want your players to experience? And is there perhaps a good reason why the wizard might not want to travel there himself? Think about dangerous terrain, and environments that affect the player's efficiency in some way (such as a foggy swamp, or an active volcano). But make sure that this location fits with the region.
Also try to think about what choices the players may make along the way. Do they have a good reason to accept the wizard's quest? Do they have good reason to trust him? What if they don't? Can the wizard convince the players of his good intentions? And what if they betray him? What then? My golden rule for this is, if you introduce a villain, be prepared to have him be killed by the players that very session. Can your campaign recover from that if it happens? If it can't, think of a way for the plot to continue without the villain. And never make the villain automatically escape! That is cheating, and your players will hate you for it. If you introduce a villain, his life is forfeit.
Lastly, if you have a plot twist waiting for them, think about how likely it is that the players guess the twist. If the wizard turns out to be evil, that is very cliche, and the players will probably anticipate that (I would assume the wizard is evil for sure). You can play with such expectations, by having the exact opposite be true. You can then purposefully feed the players red herrings to reinforce their false beliefs, to ensure a big surprise at the end. Or, which I do all the time, make the villain kinda evil, but also kinda good. I like having villains that are a bit in the gray. Sometimes the players may be put in a situation where they have to work together with the villain to fend off a greater threat. And maybe the villain has good reasons for his evil acts, that make the players empathize with him. It's always a good idea to sprinkle some doubt in the minds of your players. Also keep in mind that villains can switch sides if they have good motivation to do so.
