With a cool website (www.acoolcave.net), the Crystal Cave (wiki) was discovered in 1881 and is Wisconsin's largest underground cave. Almost enough to make a spelunker out of you!
This is the site where they voted to make Madison (being on an isthmus helped the voting, but the old traditional political persuasion tactics of bribery didn't hurt either) the state Capitol.
While the evidence appears to be more concrete for the traditional crucifixion site, Gordon's Tomb is an amazing place to see what the area looked like back in Jesus' day (as opposed to what it looked like after the churches were built).
Geographic Nerds - get ready for the (almost) exact center of the Northwest Hemisphere - allowing you to join the prestigious 45x90 club! And sign the guest book in downtown Wausau to get your membership coin!