Here are some ideas to get started.
Choosing your project:
Find sources that help you understand how the various devices or systems work. This can be books, websites or even visiting a lab where they might have a more sophisticated version of the project.
Identify the various components of the systems and their function. Usually there will be optical and electronic components.
Once you have a good idea of how the various systems are supposed to work, brainstorm with your teammates to get practical ideas on how to design them. Be creative but follow the physical laws too!
Out of all the possible designs that you and your teammates can think of for the different projects, choose the two that seem more simple and inexpensive to implement.