I second the inline skates option - I actually cycle for my skating, and it's a great cross workout. Also, if lugging around skates in not an option, some gyms have slideboards you can use - basically sheets of waxed plastic that you "skate" across using little booties over your shoes. It's very tough - best done in intervals. You can also pick one up for not too much money...I've had friends make them out of old kitchen countertops
