I actually just wrote a hub about it called "Wanna Homeschool Your Preschooler? Here's how!" Of course, you don't need to be homeschooling to do the same activities I talk about in the hub.
My biggest recommendation is to read and talk about the book as you are reading it. Talk about the pictures, even the parts that aren't directly related to the plot of the story. Like in an illustration of a girl walking along the beach, you might notice a seagull diving down to the water. Talk to your toddler about the bird and what it might be doing by the water and where it might be flying to.
Pull out pots and pans and let your toddler play the drums or stir. Crayons and paper are fun, cut up colored paper. Toddlers love to explore, so any mess you are willing to let them get into, they will love!