Big bowl of flour with some scoops, sit him in the bath and let him get messy and play.
save the flour and next time make cloud dough, 1 part oil (sunflower, olive etc) to 8 parts flour. It's mouldable but crumbly and a lovely texture to play with.
Drawing with crayons, just give one or two crayons so they don't get overwhelmed and practice drawing.
Set down a big towel and put a bowl of water with scoops and things to 'wash' add baby bath if you want bubbles.
Let them help you sorting out cupboards, washing etc, let them pass things to you, or give them a pile of things to play with.
Get them involved in the housework, give them a duster or get them to help tidying.
Go for a walk, take time to point out the flowers, little bugs, trees, houses etc. Go at toddler pace and let them explore and see new things..
Take them to a sensory room at your local soft play.
Play with blocks, read books, sing nursery rhymes and teach them the actions.
Look into going to toddler groups for brilliant stimulation. my local library does a free nursery rhyme group which is fantastic.