I shopped at my favorite "cheap fun" store: Dollar Tree. They sell oatmeal in 1lb containers for $1 each. I bought seven to add to the one I had at home, hoping that eight would be enough to sufficiently fill the bin. It filled the bin not quite half full. I also purchased three new construction vehicles; one for each boy - just to make the day a little more special for everyone.
The original instructions come from a blog called Snails & Puppy Dog Tails; she has a nice list of boy-related sensory bin ideas that are budget friendly and holiday/season themed. Her directions say to add 1 cup of water per 1lb container of oatmeal. I skipped this part. First of all, Jonah doesn't like messy play. I could envision him taking one swath through the "mud" and being done; wanting his hands cleaned. Secondly, once water is added the shelf-life of the oatmeal is shortened. Not adding the water changed the bin from a one-time activity to multi-use over a longer period of time. We managed to keep the bin out (lid on when not in use) for about a month. This gave Jonah the opportunity to play in the soft oatmeal once or twice a week.
The oatmeal is a fun, soft, edible medium to play with. The boys were able to easily load dump trucks and back hoes. They also enjoyed burying & un-burying cars. One word of advice: the vehicles will get "dusty" and you may wish to rinse them off when you're done.
When Jonah was finally finished playing in the oatmeal, we transferred all of it into a smaller lidded bin and stored it for later use. At some point, I will probably try adding water and see what Jonah thinks of playing in the oatmeal "mud."