Here are some ideas to use in designing a sunflower themed unit. Enjoy!
Bring a sunflower head to school and have the kids estimate how many seeds are in it. Take out the seeds, group in tens and count.
You could take the paper plate flower idea and put the kids photos in the center for a cute wall display on the outside of the room. Welcome to "Kindergarden" could be your caption (except so many people spell it that way anyway, don't know if I want to add to the confusion, LOL)
You could measure VERY small items with both shelled and unshelled sunflower seeds and see what the difference is and discuss why
You could do some research on the net about different types of sunflowers and compare their sizes. You could find out how many kids tall certain sunflowers are.
Kansas is the "Sunflower State" find Kansas on a map and talk about it!
You could make "sunflower jello" by making yellow jello (and talk about rhyming) in a clear plastic cup and put peanut butter or chocolate chips in the middle to form the center.
You could give them brown circles of const paper and let them put a gold glitter happy face on it, then make yellow thumbprints all around and add a stem and leaves.
Plant sunflower seeds and observe in class.
Eric Carles book, The Tiny Seed would fit in well, I would think.