There's probably a lot of different things actually.
-Bottles and sippy cups
-Coloring books and crayons
-Animated DVD's, especially the older releases you can get for about $5
-Small toys, like Hot Wheels cars, My Little Pony figures, etc.
KimbaStarshine had some great suggestions, as did everybody else. You could also include some cheaper/smaller things like candy, stickers, or those little tubes of bubbles. Maybe you could put together a little gift basket, even build it around a theme: throw in some candy, a coloring book (or print some coloring sheets out!), some crayons, some stickers, a cheap toy... ?