Oh, toddlers. They just love to dump and pour. X's favorite items to dump and pour are his sister's markers and crayons ... and anything else tiny. I set up a simple sensory bin for him with a small layer of birdseed and two different sized cups to dump and pour to his heart's content. One of the cups is a plastic container, so it's clear. He really enjoyed being able to watch the seeds fall into the container as he filled it.
And of course it is always SO fun to dump, dump, dump!
He closely observed the seeds as they fell into the container.
Depending on the age of your toddler, it this bin can be great reinforcement for simple vocabulary like "in" and "out" or "dump" and "fill." If you have two different colored containers you can practice those two colors as well! When X was done, I scooped some of the scattered seeds back into the container (and poured them back into our bag of birdseed) and vacuumed the rest. A lot of entertainment for a little bit of set and clean up.