I was asking a friend what she did for nature study and she gave me some ideas which I will share, with due credit to my friend who is another veteran hser and a mom of six:
First, we both have older children who will be following the Mother of Divine Grace Natural History syllabus so she suggested starting with that as a sort of general spine theme for the whole group of kids (she is way, way better at multi-level teaching than I am).
Go to the Forest Service or tourist information center in your area (we both live in National Forests surrounded by spectacular scenery) -- pick up any free brochures and bring them home to generate ideas about possible field trips.
Visit a couple of selected spots at regular intervals during the year -- building up a relationship with them, take photos and then sketch at home. See if there's any notable art based on the scenic spots. You can bring your sketchbook to the scene, but we agreed that this was very difficult for us during the many years that we had little ones. But drawing at home based on photos taken is a nice alternative, and you could even use the photos and sketches to make a natural history scrapbook.
Pick a few nature writers to study -- some examples we came up with are: John Muir, Rachel Carson, Thoreau, Audobon (her ideas) -- (my ideas)Beatrix Potter, Donald Culross Peattie, Edwin Way Teale, and JH Fabre (a family favorite).
Once you have a few selected, you can read a few of their books alongside with a well-written biography or documentary of their lives
She has a kindergartener who loves to color, so Prismacolor pencils and Dover-type realistic coloring books can come in very handy with the littlies.
There you have the embryo of a nature study plan that includes all the age groups.
In addition, I realized we have a Sierra Historical Society bookstore in our area that has various local history and nature books and resources on sale. It might be nice to develop an intimacy with our local region this year -- it is really the first year we haven't had various issues that prevented us from exploring -- for several years it was pregnancies and babies, then medical difficulties, then last year our car was dying slowly and we didn't trust it on the backroads and byways.
Finally, more on the science proper front; Kieron seems to love doing those simple experiments and projects and crafts, so perhaps we will build on that this year.