This is probably one of my favourite themes. I love the looks on little faces when you do something cool. We didn’t do crafts but instead did a few science experiments.
Lava lamps
We used a glass container we had here but I’ve done them at home with my own kids and just used a small pop bottle.
Fill the bottle about 1/4 of the way with water
Fill it the rest of the way with cooking oil
Add food colouring and glitter (optional)
Drop in an Alka Seltzer tablet

Elephant toothpaste
Mix 2 tbsp warm water with 1 tsp yeast in a container that you can pour
In a pop bottle, add 1/2 cup 6% hydrogen peroxide, food colouring and 1 squirt dish soap (Note: Most pharmacies around here don’t carry 6%. I used 3% and it still worked)
Pour the yeast and water mixture into the bottle.

Ooblek
1 cup cornstarch
1/2 cup water
Food colouring (optional)
2:1 ratio