When I decided to give pre-k homeschool a shot this year, I knew I did not want to spend a ton of money. And you can drop A LOT of money on books and curriculums designed specifically for homeschoolers. A LOT! The only subject I bought a full curriculum for is reading (I had no clue how to go about teaching someone to read). Everything else, I'm just kinda winging it. For science, I am using this book called What Your Preschooler Needs to Know. It's certainly not a complete school curriculum, but it's working great for our purposes.
We read a chapter (which is really only a paragraph or two) once a week, and then read library books and do activities that correspond with that chapter throughout the week. Right now we're working on animals. We just finished learning about insects and spiders. After learning about them, we of course had to make our own out of pipe cleaners and egg cartons and go on an insect hunt in our backyard.
I know I'm supposed to be the teacher, but I'm learning new things, too. For instance: holding a giant grasshopper with your bare hands does not make a 4-year-old squeamish a bit. In fact, a 4-year-old may even think it's "so cute" and want to keep it as a pet! :)