After you teach respiration and fermentation, use this lab. It allows students to perform a classical yeast fermentation experiment but specifically has them collecting qualitative data. It emphasizes experimental variables, graphing, data analysis and experimental design. A final component of the lab allows for students to design their own experiment for more inquiry style learning. This could easily be turned into an end of unit project if time allows.
I have done these experiments numerous times with my Biology and AP Biology students. The creativity that they bring to the inquiry lab always blows my mind. I have enjoyed having them present their findings to the class, as it is a good opportunity for them to learn to talk about science concepts to an audience.