Put real spiderwebs on paper. preserve a spiderweb on paper to explore/look at. or put the black paper behind it so it can be seen in detail and then take a picture so you don't have to destroy the spider's web.
Making potions is an all time favourite. and so easy to do, Plastic bottles and funnels and plenty of water and powder paint or food colouring. This activity can cover almost every area of development (Pour Water Plastic Bottles)