A workout using a stationary bike is a great way to bicycle without being outdoors. There are two types of bikes: upright and recumbent. You should choose which one you will ride based on your unique circumstances, abilities and goals. Larger gyms will have both types available but either one can provide you with a great workout.
To get the best results, it is important to make sure you stay above a certain speed and tension level. You should challenge yourself to stay above a certain rpm for a predetermined amount of time. For example: If you ride while keeping your speed above 75 rpm instead of just riding without watching your speed, you will get a better workout. Try to avoid the mistake of reading a book while riding like many people do. Chances are if you are pushing yourself, reading is the last thing on your mind.