One way would be to consider what you're most interested in * how much time it will take, and try to optimize both. I like to work on two side projects at once

One way would be to consider what you're most interested in * how much time it will take, and try to optimize both. I like to work on two side projects at once (split by morning/evening): a long-term project (e.g. paid product/SaaS/book/etc) where it's hard to see progress everyday, and a short-term project (e.g. blog post/infographic/etc) where progress is easier to make. The reason is: I find that being able to see daily progress is important to keep you going with side projects in general.