This podcast describes 4 things that teachers do that kill discussions.
I also describe one of my favorite creative writing assignments that you can use on any given day. You will also see how quickly you can adapt it to come up with multiple creative writing assignments.
Please do not think that just because the students have already done it in the past that they will not do it a second time with slight variations. Although they might complain, if you can explain to them how many times they practice layups or taking photographs or programming a computer game (or whatever your child does over and over again) you can help them see that practice leads to improvement. Once they start to see improvement in their writing you will find that inspiring them comes easier.