Sites such as eBay allow you to sell whatever you want to sell online. It can be used, brand new or even something that you will make once an order has been placed. This provides you with an opportunity of making extra money by just selling goods that are of no use to you but can be of value to others.
Similarly, you can start a part time business which you can manage when you are free from your job. This can later transform into a full time business once you are earning enough. Websites such as etsy.com offer you to sell your homemade products which is a great idea in the given market segment.
Can you write or make graphics? Or is copy-writing an additional skill that you posses? These can get you some freelance work that can pay from moderate to very well depending on the nature of the job, the time you spend on it and your performance.
This is a great way to make some extra money while you are able to utilise your brain more, something that the doctors are now recommending you should do often. This can, like selling on eBay, later become a full time job that can pay pretty handsomely.
This is one of the more common methods that are being employed by people. A second job can work well as long as you get paid decently and take good care of your health. It is easy to start working like crazy yet get unhealthy and suffer from fatigue. The best way to go about it is to do something that does not physically or mentally tire you and stress you out.