My son loves his weekly 'treasure basket' - a low basket filled with interesting, themed objects that he can explore and take out the wonder of. I change the theme every week - one week it might be all things silky for sensory play (scarves, fabric pieces, etc.), another week it could be nature-inspired (pinecones, feathers, interesting stones).
I also love the good old toilet paper roll bin! Cut up a bunch of rolls and let them paint them different colours. Then you can help them thread them onto a piece of string for a fun necklace or a colourful garland!
Another favourite is the sticky window art. Using contact paper or clear adhesive film, create a sticky 'window' on a portion of glass - could be the sliding door or even a big picture window. Toddlers can then stick bits of coloured tissue paper, leaves or pom poms with lots of sticky fun! It keeps them occupied and also looks really pretty when the sun shines through.