Fill a box with old clocks, watches, and watch bands. Or try using old toasters and irons. Or provide motors of various types. Be sure to clip the cords so they can't be plugged in. Provide the tools they will need to take them apart.
Children also love to put things together.
Fill a prop box with odds and ends of art materials. Encourage them to explore and experiment with many different art mediums. Try a costuming center. Provide paper plates and bags for masks. Give them old clothing that is clean, including ties, belts, hats, aprons, shoes, and bags.