When I go grocery shopping each week I always pick up a few extra things for the cupboards so that I am prepared for some messy play or even science experiments when they boys ask for them. For messy play things like jelly, corn flour, shaving foam, are always handy to have in. For science experiments things like baking powder, vinegar and food colouring are always on my shopping list. I would always make sure I have a supply of flour for baking and for making moon sand or playdough. It is handy to have a stock of the list ( see below) in my cupboard so that I have it ready available for when the boys ask to do something in particular. I try as best for play especially for messy play to be child led. See post about messy play to give you ideas on how to use all the items from the list.
Other stores I get the non food items from @dealzireland @eurogiant @mrpriceireland @tescoireland they are brilliant for stocking up on things like masking tape, poster paint (Mr. Price I find is the best for poster paint) and arts & craft bits or your nearest discount store can be good for picking up resources.