I agree with the fact that asking your students what they want to write about is a key element in teaching young students. Young children have a lot of imagination, so why not have them use it!
I have some examples that might help you. You could read a story to your students and have them invent the ending. You could have your students create their own comic book or their own pop-up book, which in my experience worked very well. In the summer time, have the students go outside with chalks and write a poem on the pavement. Another great activity is to have students, in teams if four, write individually something on a given topic, afterwards they come together in their team to make it fit. You could also do a guessing game, where students write about themselves and the other classmates have to guess which student wrote the text.