What makes for a great work day for you?
"N hours of coding"
"Yesterday I told myself I'd add X feature tomorrow. I did that today!" (even if it only took 2 hours)
"I ate a good breakfast. I worked out. I played with my kids. I did N hours of work and didn't question my sanity."
In vague terms, the best work days are when I start, build, and complete something for myself. It could be a prototype, a one-off program, or a feature on an existing application. It's a milestone of encapsulated accomplishment. This is especially nice when it has a visual component I can point to and share with someone. If it's for my employer, that's ok too, but not as fulfilling.
There are other seasons when success is just attending work, making a small step of progress, and not quitting.
Unproductive: working on a presentation that is unrelated to what I work on, having a full day of meetings that don't move the needle
It took me, literally, years of therapy to accept that scheduling my days will not limit my freedom and creativity. On the contrary, it gives my rational side sovereignty over emotions and makes typical day a safe space to focus all creative energies on a particular task.
Have not skipped or delayed a meal, avoided distractions which came my way as I was deeply focused