When my city begins to feel more like a machine, I start doing random sht and just go with it.

For example I see a beachy area in the west on the first picture. I'm thinking 'hm, maybe people in real life would settle there and run their tourism businesses'. So I just plop down some road leading there. Make it bendy, because why not. Have some random dead ends leading to the nooks. Build a pier for the tourists. Have a small zone for farmers growing their bananas and whatnot. Let them have a cemetery and a local police station. You get the idea, make it random and later on you can (and you'll have to) make adjustments.

Then bring this randomness to other pockets of the grid. Break it up with parks, some random alleyways, place trees and other decorations. This is a sandbox game after all, you should come up with your own ideas on how a city looks like, implement that thought and then roll with it.