This is my favorite engineering project, EVER. It worked wonderfully. The students were engaged every step of the way. It was accessible and easy for them to evaluate their success. They were motivated to improve their designs, and for the more artistic-minded students, the criteria to be attractive gave them an outlet to incorporate aesthetic elements in their design.
This project works as a team or individual project. It works for a wide range of ages, and there are several opportunities to extend this activity. For older students, you could have them do a second project to design a propeller. For this you would need skewer or straw to move the motor away from the water, and students will need to investigate the difference in size required for spinning in water. Another extension would be to look at different types of fans, like centrifugal fans, and to try a similar design.
The engineering criteria and constraints are included in the directions, and you are free to develop or refine those with the students before they begin.
This is a fine engineering project to go with a unit on electricity and magnetism. The materials, including motors, batteries, and test leads, are commonly available. Having a variety of batteries, and therefore voltages, allows the students to vary the speed of the fan. Older students could design multi-speed fans.
There is an opportunity to tie in this project with an art class as well. Colors on a spinning fan will blend and allow a discussion of primary and secondary colors.
Note the first criteria that the students' fans must move air "in one directions" is key. They will be excited, at first, to feel the moving air around their first attempts, but when they check to see if any air is moving in a direction, they will realize the challenge. You can set up a method to measure success, such as moving someone's hair, or moving a strip of paper.
I have listed this project as lasting 3 hours, but you could definitely use more time, and maybe, depending on your students, a little less. Enjoy!