I go in waves to be honest. I do less planning in the summer as we spend so much time outside but I the world yet I find it helps to have some kind of plan to help prevent cabin fever during the colder months. In addition to your Montessori program you could have a daily adult led activity where the focus is on one group activity. Might be as simple as a collaborative activity focussing on one type of toy versus every child having something different out. I do this with a toy that gets less attention but works well as a group. For example the megabloks. It teaches the kids to work together, focus rather than going from one thing to another in a few minutes and forces them to play outside their comfort zone encouraging them to have new experiences.
I will also do some themed activities but I have a habit of doing things I like more often than not so I do try to push myself every year to explore other things. For winter themes I will die cut mittens from scraps of material, add numbers or letters for each child's name, string them into garlands, make collages etc Pinterest is an awesome source of inspiration. This year I plan to do paper plate crafts more and also the crafts that go with a popular children's book. I hope this helps :-)