If you're looking for a fun and educational way to spend the day, Wonderscope Children's Museum is a good bet. Wonderscope is housed in an old elementary school, and each classroom is full of hands-on activities that fall under a distinct theme. One room is set up like a hospital, one is grocery store, one has a nature theme, etc.
The nice thing about Wonderscope is that it's fun and novel enough to feel like a big-deal outing, but the relatively small size of the facility makes it very manageable for families with small children. There is even a room designed specifically for kids two and under to keep the toddlers happy.
Wonderscope can get crowded, and therefore less enjoyable, when a school field trip is there, so it might be worth your time to call and see if they have any large groups booked before you go.
Admission is $6 for anyone over 3 years. Kids 1-2 are $3. If you plan on going regularly, a membership is worth your money.
Visit www.wonderscope.org for more info.
Click here for an interactive map of all Free State Family destinations. 
