I would use the activity set: Space and Aliens.
I would start by using a story ‘Aliens in Underpants Save the World’ I have read with my own children, who loved it. Read Aloud Kids Book: Aliens in Underpants Save the World by Claire Freedman and Ben Cort (youtube.com)
I alternate between reading the books or using a nice version on YouTube as I find that the children can all see the pictures better using the interactive whiteboard and it also gives them a break from listening to my voice.
From this story we can discuss what we think aliens look like: colour, shape, features – eyes, mouth, hands, feet etc.
Next the children will draw/design their own alien – some might prefer to draw their own alien using markers and crayon while others can use straws, string, button, wool etc. or they can use both.
Next is the important part as they get to name their alien and write a small bit about what planet their aliens are from and how they live their lives: is it hot/cold, what type of house they live in, what job do they have, how they travel around, where do they go on holidays?
To finish, the class will present their alien, tell us their name and how they live and the other children will get an opportunity to ask questions.