Elementary education is about creating a child's desire to learn.
Granted, elementary students learn how to write, read, and do arithmetic as well. But I truly feel the most important thing an elementary teacher can do for his/her students is fill them with a desire to learn. And astronomy is a great way to do this.
Here are three projects that will get students involved not only in astronomy, but in science AND learning as a whole:
1) Count the stars
This is a project aimed at K-2 aged students. At the end of class one day, challenge them to go home and count the stars. This project is worth it immediately when you hear their responses. You'll hear every thing from 4 to a gazbillion zillion million.
The project, of course, isn't about the results your children come back with. It is about the fact you have had them look up at something thy cannot begin to understand or grasp. After asking them each how many stars they counted, tell them that there is actually no one that knows, and that any of them could be correct. This will not only allow them to first realize the fact that there are questions people do not know the answer to (it seems obvious to us adults, but kids don't usually understand the unknown) but it will also allow them to feel as though they have the chance to do something no one has done, which almost always gets them interested in the subject. From here, explain that the sun is actually a star, just like all the ones they counted, only it is closer. This is a fact that they will never forget. Use the fact that nobody really knows how many stars there are as a project for them to come up with other things that nobody may know. It is a really good chance to open their eyes to the fact that they could be the first to discover something.
2) Solar Eclipse
This project is aimed at grades K-8. The reason this project works is the fact that it doesn't make sense to the uninformed mind. Why is the sun being blocked out? For older students, this is a great way to explain our solar system, which allows you to then explain the galaxy and universe as a whole. The pinhole method has been a favorite for many years as a way to view the eclipse, but I encourage you to take advantage of the technology we have. Give your class this assignment: What is the best way to view the solar eclipse without looking directly at it? You'll be surprised with what your students come up with. Digital camera's and video recorders are a safe way of viewing the event without risking one's eyesight. Let your students figure that fact out though. Some will come up with really smart ways to capture it. Then, use the information you received from their ideas to map up our solar system, and introduce them to the small spec our solar system is in the universe.
3) Define Planet
This is a project for kids grade 8-12. Define what a planet is. The fact of the matter is most of us teachers would have difficulties coming up with a concrete answer. One thing I have done that has really worked is put a challenge to my students. I tell them that I grew up knowing Pluto as our ninth planet, and most of them (at this point) grew up under the same beliefs. At this age, it takes something more than wonder and star counts to get at students. So I suggest you start the 'Save Pluto' campaign. You have to do some of the ground work (finding out the qualifications for what a planet is, and making it accessible to your students) but then you leave the rest up to them. Group them up, and have them present to you, and the rest of the class, why Pluto should or should not be considered a planet. One thing that is key, is that you allow them to have an amount of fun with this. The fact that Pluto, all of a sudden, has been decommissioned as a planet can be funny. Allow you kids to explore the humorous side of this issue while also requiring them to support their claim. The end result has your students studying the other planets of our solar system to draw comparisons and difference to Pluto.