Toddlers - Chicago Children's Museum (and Navy Pier)

Toddlers are a great age for enjoying the Chicago Children's Museum. I'd recommend Kids Town, Dinosaur Expedition, and Treehouse Trails the most, along with Play It Safe (the fire truck) for older toddlers. Younger toddlers especially may like the Pritzker Playspace as well, though hours are limited and big siblings may not be allowed, so follow that link for more information. Water Ways is also a big hit, though the water is up high and you may need to hold up your toddler a lot in that room (and bring a change of clothes if you plan to go there). All of those rooms, except for Play It Safe, have a single entrance/exit, so you can give them a little more space as they get more independent.

Toddlers just learning to climb stairs will enjoy the stairs on the fire truck, but only if the museum is not very busy because there's not a lot of space when other kids climb around. Once up to the second level of the firetruck, there is a way to come see you, and walk around independently which toddlers love. There is a third level of the fire truck, but toddlers cannot get up there without a lot of help (there's a big ledge that must be climbed up to prevent toddlers from even trying).