My son has an obsession with Monster Trucks - so I figured using them to identify (and mash) letters would seem much more like play than work. Today we focused on the letters of his initials.
After labeling each truck, I drew corresponding letters on the bottom and sides of brown paper lunch bags. [Tip - when writing on the side - be sure to write close to the bottom of the bag as you will lose some room at the top when you seal it off.]
We blew up the bags and sealed them enough to hold the shape (but not so tight that we could not easily refill and reuse them).
Then we mixed them up and placed them on the floor.
I made sure my son could first identify each of the letters on the bags, then established the rule that each monster truck could only "mash" its corresponding letter.
Once all of the bags were completely flattened, we blew them back up and played again, and again, again. This activity was certainly a "smashing" success.