Spinner Bikes, Exercise Bikes or Stationary Bike are great way to get fit indoors. It builds stamina and endurance while shedding a lot of calories. If you go to a gym, I'm sure they have an Stationary Bikes. It's really easy to use, but make sure you are doing it right and have the correct position to avoid injuries.
GCN has created a Youtube video on how to use Spinner Bikes, Exercise Bikes or Stationary Bike correctly.