I was recently emailed these great lesson ideas for those looking at space from none other but the good folk at NASA's education themselves.
Let your students design their own space mission and then let them share their ideas with others. This would be mostly suitable for younger students.
Your older science minded students might like to take a look at Creating a Comet Out of Dry Ice by following the video tutorial. And there is a craft version of creating a comet for the younger students that is available here.
If comets are not your thing then you might like to try and make a moon crater.
All in all this is a great resource for teachers and students of all ages that are looking at space and I would highly recommend taking a look at it.