The Henry Vilas Zoo is a Madison treasure! It is a free zoo--a FREE zoo--meaning free admission and free parking. You can go there and not spend a penny! It's small enough to be kid friendly in that you can see it all in just a few hours. They are constantly updating and improving their areas and exhibits, and they offer amazing educational programs for school groups.
It's not only good for kids, it's also a perfect place to go on a date. It is small, there is lots to see, its public, and its just a short walk from the UW campus and the state capital. Plus it's surrounded by Wingra park, so you can do a stroll through the zoo followed by a picnic in the park. The perfect blend of casual and yet intimate.