All you need is a large bin or rubbermaid tub, a few toy trucks, a shovel and a package of bird seed. Your toddler will do the rest. The best part is that anything that gets spilled out will be eaten by the birds!
For indoor fun, you could try a tub filled with rice or beans. I will warn you, the rice is hard to clean out of carpet. Just don't try the rice outdoors, the birds will eat it but it's not good for them.
And if you like milk, here is another great idea - save your milk lids (or better yet, ask your friends & family to help you) and put those in a tub with a shovel and a net and you have another fun indoor activity for a rainy day.