I would choose the activity set on the sun and shadows. There are so many different ways of exploring the sun and how it affects shadows. I like using chalk and working outside. We pick certain items in the yard that cast a shadow first thing in the morning and trace around them, I also draw the outline of one of the children. We come back out three more times during the day and trace the shadows of selected items in a different colour, the child must also stand on the same spot! Using the different coloured chalks really allows children to see how the shadows have moved.
Another lovely activity that ties in well here, is shadow drawing. The children select toys (ideally little figures e.g. dinosaurs, farm animals) and trace their shadows on to paper with pencil.
I also like to do an experiment about where do the stars go during the day from mysteryscience.com, a very simple and effective lesson.
The Sun Activities booklet is a lovely resource with some great ideas, I particularly like the idea of cooking outside!