Hoping to get some advice on how you would lay Pebble/gravel (is it advisable) on this stretch of concrete around the planter boxes to make it look nicer?
I can't see any reason what you couldn't lay pebbles straight on top of the concrete there. A couple of observations are that the pebbles might try to enter the drainage grate and they will have a tendency to end up in the shed. You could add some garden edging between the concrete slab and the drain and a strip of timber run in front of the door to stop them going astray. Apart from that, I would think it's just a matter of pouring out the pebbles to a thickness that you can't see the concrete through them.
I look forward to following along with your project and please let me know if you have any questions.
Please keep us updated on your project and we would love to see some pictures when you are finished.
Let me know if you run into difficulties or have other questions. We're here to help.