Wisconsin has converted an impressive number of abandoned railroad lines into paved, bike-only paths. They go up hills, through old tunnels, over bridges and alongside pastures. Wherever you are in the state, there's likely a sweet ride nearby; check the Wisconsin Biking Guide (available at www.travelwisconsin.com, under Planning Tools, then Order Guides). The 400 State Trail (www.400statetrail.org) and Elroy-Sparta Trail (www.elroy-sparta-trail.com) top the list.
Bike rentals are available in gateway towns, and you can buy trail passes (per day/year $5/25) at area businesses or trailhead drop-boxes.