When introducing a new structure, I always like to introduce it with something fun. This takes less "work" for students so we can focus on the how-to-steps of the structure rather than the content. After awhile, even if students know the structure well, I like to throw in something fun for a change.
Also, I never introduce more than one structure at a time. Depending on how well my students do with the structure, determines when I teach a new structure. I have found, like teaching anything new, it varies between each group of students.