I like the idea of linking some activities around an area such as Aliens. I would start with a story to engage them such as: Here come the aliens.
They could then make the alien a welcome drink using components such as: baking soda, food dye, vinegar, washing detergent and water. By combining these substances, the children could see the cup fizzing and foaming.
I could adapt the mice rocket to be alien rockets experiment. I would include key questioning as we could have a competition about whose alien goes the furthest distance and which alien goes the highest in the air. We could see if bigger or smaller bottles work better and which size of alien works best.
Another experiment which I have previously done would fit in well with the theme would be finding a rubber egg (alien egg experiment). The children would have great fun examining the egg and seeing what is happening. An alternative to this is putting the egg in different liquids and examining the reaction:
http://www.nickarnold-website.com/experiments/alien_egg_experiment.pdf
Aliens could be given skittles but first they want to put warm water on them to eat them in a different way. Another experiment which could be incorporated would be the magic milk experiment where milk, food colouring and washing up liquid are combined. Children can make predictions, carry out the experiment and discuss the findings and begin to say why something has happened. They may start to use prior knowledge to help predict also. They could suggest alternatives to the experiment or amendments which could be made.