I’ve come to start enjoying side projects as truly secondary activities, without even designating specific time blocks for them. This lets the idea gain maturity in one’s mind, and allows an extensive line of thought before action is made. It allows longer iteration cycles, which might be what separates side projects for non-side projects, product-oriented work, instant specification, real need (we’d think of it as…tasks? in such cases).
It lets the project be carved with our constant personality development and be nourished with develop knowledge chunks that occur with the passage of time.