Calling for linear regression application ideas

In this semester of my PhD education I am taking a course on Project Based Learning (PBL). The major outcome of this class is that I will design a PBL unit. I know that I would like to do this unit on linear regression but I still need help.

I would like to be able to use the project outcomes to help a business, school, or community. The students will be creating a (hopefully) accurate linear regression equation that can be used for prediction. I am looking for ideas of variables that are believed to be associated (e.g. height and weight) that can be researched data can be collected and an analysis can be done.

I have the option of having the students work in groups which means that I could have them each look at a different relationship. The other option would be to have the whole class work on the same relationship and work individually.

No idea is a bad one so please feel free to reply with any brainstorm that may arise.

Thanks ahead of time!