Congrats on starting your own daycare this summer.
I think it is great that you are being pro-active but have to agree with the others who said it depends on what ages you will be caring for. Most kids don't care about circle time very much until around 2 years old. The 18 month olds LOVE songs but they don't care or understand weather and 2 year olds can't really understand weather much yet either. They might understand snow and rain and that is about it so for days like rainy days you might want to sing Itsy Bitsy Spider and Mr Sun and Rain rain Go Away and that might consist of your circle.

I find the best programming for mixed aged groups is to base it off a book. Ie. The Very Hungry caterpillar and you do art time painting large caterpillar cut outs, you shape noodles like caterpillars for lunch and use fruit from the book for snacks etc. Do a sensory tub with spaghetti noodles and hide plastic food inside it for the kids, etc
You could do themes with Knuffle Bunny, Robert Munch, themes on leaves in the fall where you go out and do simple stuff like collect leaves and make leaf bouquets. Keep it simple because it really is hard to appeal to young and older kids with varying degrees of attention and grasping a 'theme'

I also agree with Kangeroo Mama that kids LOVE the Circle time at Early Years where it is a large group of 10+ kids whereas they really don't enjoy it much at the house. I do one of two quick songs when we do circle at home and then read a story during the circle. I might talk a bit about something off tangent and educational depending on what comes up or use the time to have a 3 yr old tell me something they did last night etc. Be prepared for the 12-18 month olds to go running or crawling out of the circle and that is alright. That is just how it is at that age