This is a great thread. I am sure all of us at one point in time considered this very idea. I know I have and I am always researching potential "side" businesses. First off, I am currently a business owner and have been for nearly 7 years now, currently I work close to 50 hrs a week. I own a medium size supermarket in the metro detroit area. By no means do I consider myself an expert on independent business ownership. That being said I am currently working on establishing a side business. For me it is buying salvaged sport bikes, rebuilding them to a mechanically functional state, and selling them. My potential market are the racers and track crowd so cosmetics is not an issue. I chose that activity for several reasons. Hopefully this will give you some insight on my decision and also help you make yours. In my opinion it came down to what characteristics I considered most important. They are Passion, Capitol, Convenience, Prior knowledge, and Time management.
Passion is obvious, if youre only slightly interested in anything leave it alone.
Capitol, what can you afford to spend. Include all aspects; initial capitol, consumables, professional fees(shipping, auction, buyer/seller fees), and tooling.
Convenience, make whatever it is convenient enough to be able to pick up on a whim. You don't want to have to drive 30 minutes from home to get to where ever it is this thing is happening. For example I work in my garage. This is why most franchises did not fit into my goal. They required me to be in different parts of the state or more than an hour away from home.
Prior Knowledge, its much easier to pick up and run with something you have experience with. Don't waste your time trying to learn a brand new to you trade. It will prolong and delay any goals you have in mind. Unless this is your original intent.
Time management, this the decision maker for me. I need there to be no dead lines, no minimum required hours, no customers waiting, and no pre-established sales goals. I require extremely flexible timing and the freedom. Due to my already demanding career and for other events that pop up in life. For example my current bike project has been derailed for 3 weeks due to the birth of my first child.
I hope this helps. I apologize for being long winded with this reply. I am sure some will disagree and other will agree. Good luck to you and keep us updated.