Let them sweep up some small objects deliberately left on the floor with a broom or brush.
Let them soak up some water that you've deliberately spilled somewhere with a cloth (for example, on a baking tray).
Take a piece of polystyrene and let them try breaking it to pieces, placing small objects into it, and so on.
Drop various objects into a bowl of water, and see which ones float and which ones sink.
Pick up very small objects using tweezers, and transfer them from one container to another.
Place a drop of liquid soap on a plastic plate, and let your child turn it into foam using a brush or whisk.
Fill a used egg carton with various objects.
Entwine a piece of thread or string around the teeth of a comb/hairbrush.
Gather up all the plastic containers in the kitchen, and place them inside each other from the largest to the smallest.
Sort various pieces of cutlery or other kitchen items (nonsharp ones, preferably plastic) into different groups depending on size, color, function, and so on.
Build a tower out of plastic cups.
Place some objects inside a large bag, and let your child search for them and pull them out.
Place some clothes pegs on your child's sleeves or a fold in their clothing and encourage them to take them off.
Offer a stack of colorful Post-it notes, and ask for an art installation on the nearest wall.
Unwind a spool of thread/a ball of wool.
Give your child a small amount of cotton wool, feathers, or other light objects and encourage them to blow them out of their hands.
Cover a Q-tip with a small amount of shaving foam, and let your child draw with it/mix it with water in a bowl.
Fill a balloon full of water, and let them play with it.
Draw something on a balloon.
Give your child a pile of buttons, beads, or other small objects varying in size and color, and encourage them to sort them into piles according to these (or other) categories.
Make a 'developmental rug' for your child: take a piece of thick material and sew various bright buttons of different shapes and sizes onto it. This will be of interest to even the smallest children.

100 simple ways to keep a child entertained when you’re busy