Warning: This activity isn't for everyone! Only do this if you are ok with not having your groceries put away right when you get home. I wanted to stop about half way through, but at that point it was too late.
As you are unpacking groceries, give your child a chance to group them by letter. For example, all of the items that start with B in a pile (shown above).
This turned into a huge vocabulary opportunity that I wasn't expecting. B wanted to put an N on the noodles but the box said penne, the bones said rawhide, etc. So that was confusing.
What I had in mind for the activity was the top picture, but B had a different plan (shown here). He wanted to label each item with a sticky note and forget about piles. (Notice the Fiber One was labeled B for bar.)
If piles or sticky notes aren't for you, you can also circle the letter that it starts with and talk about it as you put the groceries away. You can also hunt for numbers and try to put the groceries in order from 1-10.