About biology: Feynman experimented on ants learning the shortest way to sugar (if I remember it right - 'Surely you're joking, Mr. Feynman!').

There are commercial products based on pheromones used to call male Colorado beetles away from potato plantations (and tales of horror about people not reading the instructions beforehand.) Also, the variability of the black spots on its pronotum is a cool illustration of well-defined phens.

There are detectors that allow you to hear bats (I don't know how much they cost.)

There are cheap ferns (Carolina Biological Supplies, for example) for high school teaching about life cycles, sexual reproduction etc., including induced mutagenesis. They also sell the growing medium.

There are lichens to measure growth rates of, and there is at least one manual on growing mosses indoor, and you can experiment with adding nutrients to the pots and see for yourself that some species don't tolerate excess organic etc.

There are heaps of things to observe. You just have to know just what you are looking for.