As an early childhood teacher, I have lots of ideas, but if you don't want mess or the children to eat the craft stuff then I would say there's ot much you really could do with them.
At 18mths, children are very tactils and like to explore by touch, especially orally. I would say buy edible paints, or better yet make you own with corn flour water and food colourings, make your own dough/goop and do it all outside on the grass at a table, or on a verandah that can be hosed. Be prepared for them to eat it and get it all over them, that's the best bit for babies!!!
Smiles4u has great ideas with the water on concrete, sketch-a-doodle and chalk on concrete.
If you have to be inside, do it on tiles/lino, with a big plastic sheet taped down and have a cloth or tap at close hand and an old towel to wash them with after.