Zoo Pass

This may be cheating a little, but . . . here is a great deal on a zoo membership. A friend in my ward here told me about it. It's called Happy Hollow Zoo, in San Jose, CA. Most zoos have the same reciprocation program so with a membership to one, you can get into other zoos across the country. But the great thing about this one is that you also get into a whole list of other museums/science centers/discovery centers. Here's their reciprocation list. Scroll past the list of zoos to see the list of science centers you can also get into - looks like there are lots in WI.
Plus, its cheap.
Right now they have a "2 years for the price of 1" deal. They are closed for renovation, but still selling memberships. So I believe we paid $70 for a 2-year membership. (A membership to Seattle's zoo is $95 for 1 year).
(This is a way better sales pitch than the one I just sent out to our RS email for a free old high chair with no straps so your kid can fall out onto his head).
Also, it says on there somewhere that some of the reciprocation places listed may offer a discount instead of free admission, but we've used it at a few other places and all have been free - you may just want to check with your zoo first. There's my disclaimer.