My boys love tools and hammering. I thought we could do golf tee hammering and add an additional learning element by adding letters. I wrote upper case letters on to the bottom of an egg carton and the corresponding letters on scrap paper. You could do matching upper/ lower case, numbers, colors etc. When it was their turn they pulled out a letter and hammered a golf tee into the corresponding spot on the egg carton. We also counted how many strikes it took to hammer in the tees.
After we found all the letters they still wanted to hammer. We counted the tees and removed them. Then I flipped the carton over and told them they could hammer tees wherever they wanted. I try to balance mom directed activities and letting them be creative and make decisions.
Then I remembered I had some Styrofoam left from some packaging and I wrote some big letters on that and let them hammer the tees in to make the letters. Next time I'm going to try writing their names or sight words.
My kids are pretty familiar with using a hammer but you should supervise your child and use your judgement. A plastic hammer would work too.