Do you have Pinterest? It was pouring yesterday afternoon so we were stuck inside, putting 'toddler indoor activities' in the search popped up with a wealth of ideas.
Off the top of my head though, DS LOVES posting things. We have a little wooden post box that he'll play with for ages but you could make one out of cardboard? We also have little wooden letters but giving him anything new that will fit will distract when the letters bore him. Yesterday milk caps and medicine syringes were being posted. ☺️