This sounds like so much fun!! I might be a copy-cat…my son loves the moon and is fascinated by her, so I would start there. You could observe the moon and how it changes over time (weather permitting, obviously). Then you can talk about moon phases, what causes them, and how they affect the earth and people.
Since it’s spring you could also observe the trees as they flower and leaf out. Here, the willows are already in flower, and I think some of the alders and beeches too. These are fun, because their flowers are “hidden” and not very showy, although once you have the search image, you’ll see them everywhere. Oaks and maples will be following suit, if they haven’t started already. Oak flowers are really hard to see, but the female flowers look just like miniature acorns. Very cute. They will leaf out after they’re done flowering. Other trees will leaf out before flowering. You can talk about pollinators and how different types of pollinators (wind, insects, birds and even bats if you’re lucky enough) have different needs, and the plant accomodates those.
You could just watch the trees in your yard, or choose several from a local park/forest to follow, and make a tree journal, collecting the flowers, leaves and eventually fruits. (I’m pretty sure I read about tree journals in a recent GP…not sure if this is what they did though).
The snowdrops and hyacinths are also up, and my daffodils are pretty close to flowering. So it could be fun to also watch the progression of flowers over the course of the year. You can use this to talk about different types of plants (bulbs, herbs, shrubs and trees) and different life histories.
Sorry, I’m a botanist and former teacher, so I could probably go on, but will stop there.
Fab, fab idea!! Excited to see other’s suggestions!