This was an "activity" that Mini Me started doing on his own that I turned into a teachable moment.
While covering (and I do mean covering) some pages with stickers one day, I watched him put a sticker of a seal down and say, "/s/, /s/, seal!" We had a sheet of alphabet stickers nearby and I asked him which letter made that sound and encouraged him to put that letter by the seal sticker.
He did five or six animal stickers matched with the initial sound letter sticker before proceeding to randomly stick the stickers on pages. He had fun and I know he learned... happy kid... happy mama!