Here are stepper bikes that work your core and let you ride in an upright position to avoid saddle sores. Stepwing Stepper Bikes are available in a couple of fun models. The Galaxy 1 has 12″ wheels and a foldable design. The Titan 1/3 come with 20″ wheels and steel frame.
Titan 3 has foldable stem and 3-Shimano gears. Like what you see? You can order yours on Kickstarter.