1- As a pretext to shop for more books (to read together at story-time). There are some gems out there ... Like my recent acquisition: Monsters Eat Whiny Children.
Note: It is a kid-friendly story in which no children - whiny or otherwise - are actually eaten... I have had fun reading it (specially when within earshot of my wife who disapproves of monsters in general, and of childivorous ones in particular) but I must admit that the monsters' dialogue is a little beyond the grasp of my 2-year-old, and the black and white illustrations fail to grip his attention for the length of the whole book.
2- As musical instruments. You can use your hand on his mouth to modulate all his "words" into "indian war whoops" (You know... ba-ba-ba-ba-ba-ba- woo-woo-woo-woo...). Simple-minded? Perhaps. But trust me, it is funny! Furthermore, if your toddler finds it funny too, he will cooperate by producing endless noises. Then - provided you have modest musical skills - you can play your toddler (I can do simple tunes, like jingle bells). After some practice, of course, you will want to go demonstrate your new skills to mama (Be warned, she will just shake her head silently, perhaps allowing only the tiniest twitch in the corner of her mouth). Note: If your toddler does not find this funny he may start crying. If you do the war whoop thing while he is crying, he will likely get angry and cry even louder. At this point the war whoop - while still funny - should probably be postponed until a sunnier disposition returns.
3- As enthusiastic audience. You can dazzle them with "magic tricks" that nobody else would find even remotely entertaining (or magic). The reason for is the primary characteristic of adults (as distinct from toddlers): that they do not blindly trust everything you tell them. Therefore they assume (correctly) that you are not the great magician you claim to be and that the disappeared bunny has merely been stuffed into the back of your pants.