Milan has a great bike sharing service, perfect for short-term use (maximum of 2 hours). Bikes can be returned at any station. BikeMi is not just a bicycle rental service, but a public transport system.
Before you can use this service, you have to subscribe on the website: https://www.bikemi.com/en/service-info/subscriptions/subscribe!.aspx
If you register weekly or daily, you'll receive a user code, along with your password, via e-mail (registration on web) or via sms (registration on mobile website). The codes are active as soon as you receive them via sms or via e-mail.