This is the simplest sensory activity you can possibly do: Add some dry beans (or uncooked rice or pasta) to a heavy duty zip lock bag, zip it up well, and let your child play with it. You can add small toys or pom poms to the bag as well, if you think your child won't get upset by the fact that they can't easily get to the toys.
The bumpy texture of the beans is great sensory stimulation.
Just make sure to keep an eye on your child to make sure they don't break the bag, or to clean up the mess immediately if they happen to tear the bag open.
Psst, if you happened to come across this page and are wondering what the heck this is all about: We are doing the 52 Weeks of Sensory challenge. Come join us!