We finished our "Animals" units a few months ago but have only recently acquired some small pets for the girls to observe. Ivy has successfully raised a family of sea monkeys to the point where they are reproducing and we've found them surprisingly compelling. She feeds them herself on a carefully noted schedule. Ivy took them to her Brownies class for pet day and earned a "Pet Pals" badge.
We purchased a small fish tank and the girls actively participated in planning and choosing fish. Sadly we are down to 1 out of 3 fish but it has lead to teachable moments.