This is a overview of the requirements for a standard 5-8 grade scientific paper. It makes a great long range project.
I suggest having the students design an experiment, gathering materials, running trials, and developing a paper for one marking period. Then to continue to practice the scientific method, repeat the experiment in the next marking period, with the culiminating assignment being the creation of a science fair poster. Then as an end of marking period grade/experiential component you can have the students run a class wide science fair.