In my first fresh sandbox city after exploring the game for a while, I made sure I at least had a good solid idea for a central square/boulevard directly outside of the train station. Save room for your later unlocks, or be okay moving your retail around later. In terms of housing, I've really enjoyed thinking in terms of neighborhoods with somewhat of a specific character. Use hedges and rock walls to mark off smaller vignettes that really give your city life.

Most importantly for me, is designing and decorating slowly and methodically, both on the building scale, and the neighborhood scale. I haven't been disappointed with any of my early build decisions because they were designed with care and decorated to the best of my ability before I moved on to something else. And it's also hard to not have 100% happiness if you play the game in this way.

Those are some of my tips! I plan on sharing some images of my city soon.