As someone who has done countless side projects (check them out here: http://www.reza.codes) I suggest taking out "making money" as a variable and focus on things that interest you or something that stems from a personal need.

The satisfaction you get from these side projects will come from being able to finish them as opposed to try and make money from them. When you try to take on a side project with the goal of making money, you'll end up sinking way too much time in marketing and reaching out to possible customers as opposed to building something (which I find to be more fun and rewarding).

And the time you spend on trying to get people to sign up or even try your product doesn't have the same returns in satisfaction as building it. (my opinion of course)