Jessica Hoffarth OTR/L, occupational therapist and mom, blends those roles both at work and home. Here is a short video showing how she made a “ball pit” with household items.
This activity is suitable for all kids, including those with sensory processing difficulties.
Find basically any open container that the child can fit into and add any fun objects.
It doesn’t have to be a box and balls, it could be a pillow case or fitted sheet (and you can just hold the ends gathered together to make a loose sack) filled with balled up socks or stuffed animals.
Of course, the child should be supervised closely at all times.