The point of this activity is to see if students can figure out an English spelling rule by themselves before it's been formally explained to them.
I put students in groups of about 3 or 4 (any larger and there tends to be a couple students in each group who just check out and don't do anything) and give each group a set of 24 cards. The cards should be cut so that a word and its plural are together (like "horse | horses" or "potato | potatoes"). Have them spread the cards out on their desks and tell them that there are two groups for these cards - Group A and Group B. Their task is to figure out which cards go in group A and which go in Group B.
If any groups are having some trouble after 2 or 3 minutes, I write an example from each category on the board, like "rabbit -> rabbits" for Group A and "box -> boxes" for Group B. The solution will often spread throughout the classroom as the activity goes on, so once most groups seem to have the correct answer, check it by going through each card and asking if it's in Group A or B. I write them on the blackboard as we go.
Once you've checked all the cards, ask them why there are two groups. They should notice that Group B plurals all end in -es. You can ask them why, but in the end, it's good to confirm that words that end in -ch, -sh, -x, -o, or -s are pluralized by adding -es.