There are several theme units that are pretty popular for younger kids. In the fall, kindergarteners everywhere learn about pumpkins, apples, and the harvest. It's like there's an unwritten rule somewhere that you have to teach little kids about scarecrows and such. It's educational, it teaches them about real things in the real world, rather than abstract concepts in a book, and of course, it's fun!
If you have an apple orchard near where you live, pick-your-own apples is a great family activity. A trip to the pumpkin patch is requisite activity for this age, too. Learning about the life cycle of an apple tree might be fun, as well as cutting open a pumpkin to scoop out the squishy insides. There are lots of arts and crafts projects that go along with these popular themes, and some fun songs, finger plays, and picture books from the library would help to round out your theme unit.