Videos on all sorts of astronaut topics online for you to share with your class at How Stuff Works: Spaceflight. You might have a hard time deciding which ones to use. Lots to choose from.
Set up these learning stations and use these lesson plans to teach your class about astronaut health, training and learn about the effects of space on the human body. Some great CCSS activities you might find useful.
Train Like an Astronaut - here is a great set of activities for you to keep your kids fit and train the way astronauts do.
NASA's education site has lessons, links, projects, activities and games for you to use in your classroom. All age groups.
Spaced-Out Physiology from Arizona State University explores what happens to astronauts' bodies when they travel in space.
The Lunar and Planetary Institute offers background reading and engaging lesson plans for building a space station, launching a rocket, and designing a space colony.