I found having an ideas sheet is good for this.
You can know the idea is recorded and that removes the desire to immediately work on it. And you can later logically choose the best thing to work on from the list
Yeah, I recommend keeping a sketchbook/journal to write down and plan out the side-projects, then when you have more available time pick which one to start on given how feasible and interesting they are now. From my own experience I have found that if something still sounds interesting two months after writing it down then it's something worth spending time on.
