For Senior Infants class I would do the “Design a Martian” activity. I think this would be both age appropriate and a lot of fun.
1. To start we would look at an image of earth and describe its characteristics. What does earth have on it that we need to live here eg. air, water etc. Also describe what does it look like, how does it feel in the desert, north pole etc. I think this would provide an important foundation before thinking about what Mars might be like.
2. Look at images of Mars, what do we see, what do you think it feels like etc. Do you think we could live there? Why or Why not?
3. What might a Martian look like/need to live here? Draw a picture of what you think a Martian would look like. Build a 3D model of the Martian based from discussions and plans. The children would be provided with a selection of materials such as playdough or clay, matchsticks, feathers and beads to allow them choice in their design.
The children should be able to orally explain their choices and thinking behind their design.
Depending on ability of class or individual pupils this could be extended to write some words or simple sentences about their Martians.
I like the idea of starting these units with a story to spark interest, and I am getting some great ideas of good space themed books for children from reading other posts.