For this we need a ramp for the cars, cars, and a "oil" pit.
To make the oil pit we added one bottle of glue to a bowl then filled the bottle back up with water and added that to the same bowl. Mixed and then added little bit of black paint (could also just use black food coloring) To finished it add liquid starch and mix till slime forms.
I would keep this as a outside activity though I tried inside not even thinking of the messy it could make. :)
note~ the white little balls in the oil pit are something my daughter added later on