I have two kids (3.8 and 1), but I also have a large number of nieces that stay frequently at our house.
We have a book called 365 Things To Make And Do by Usbourne, but they have others (see link).
Things we have done over the years:
Blown and painted eggs, painted stones, made wooden spoon people, made modelling clay jewellery, sock puppets, puppet theatre using cardboard box and cardboard people stuck to lolly sticks, made pirate ships out of large cardboard boxes plus eye patches etc... And then played pirates, made masks, made dens, made a shop, baking cakes and biscuit, making pizza, made a den out of blankets and pillows, playdough (even with my older nieces when they were under 10), painting, chalk pictures (hairspray needed to fix), made perfume (bubble bath plus essential oils and food essences, made boats/rafts to sail in the bath, tried to make a water mill that will turn properly using plastic cups, done floating and sinking experiments, made badges, brooches, decorated rings.
If you are really feeling adventurous and have stairs, a makeshift slide made from single mattresses and sliding down in sleeping bags is great fun. Just remember a crash mat and to remove breakable objects.