We currently have a membership to this museum (actually quite cheap!) so we visit fairly often and it seems like there is always something new to explore here.
There are lots of options for kids - a pretend grocery store/kitchen, doctor's office, face painting, making music and (my kids' pesonal favorite) the newly re-done Water Works area.
As if that weren't enough, you can participate in story times, build with clay and other hands-on craft projects.
It is a little hard to find your way around and it is always very crowded (even when we go mid-week and mid-day), so if you can, leave the stroller in the rain gear area by the bathrooms and it will be easier to maneuver around.
There is not much room for sitting to eat, and they only have vending machines, so pack something for lunch if you'll be there a while and try to eat earlier or later than the regular lunch hour.
A fun way to spend the day with young kids!