The class wants to get a pet. This has small groups research their choice of pet, learn all about it, to determine whether or not it would be a good fit for a classroom.
The students plan out the habitat, cost out the pet, and build a diorama. They present on why their choice of pet would be a good fit for the classroom, try to persuade that direction.
This could be an introduction to studying the environment they live in for the next session or could be focused on only fish/birds/jungle/exotics, etc.