We all have hobbies. Even though we may not perceive our recreational activities as 'hobbies' - they are.
Playing video games is a present day hobby, and so is collecting a video library of all the best flicks. Collecting is a traditional hobby, and today kids rave about Batman, Sponges Bob, Spiderman, Ben 10, Baseball cards, WWF cards, and anything with extreme tacked on it.
Other huge hobby markets you might want to consider are model building, sport, coin and stamp collecting, antiques and crafts. The advantages in creating a hobby site begin with the guarantee that people will always look for these products in their spare time and they will often use the internet to do it.
Revenue will come from online sales of hobby products that you showcase in your site and also through affiliate relationship with manufacturers and distributors of the hobby paraphernalia that pertains to your target niche.
try to make some research first, and make sure that your hobby will profitable.