Iyashikei or "healing" is kind of a blurry genre and often gets applied to just about any upbeat SoL title. Meanwhile, Aria is good enough in my opinion to be the definitive example. Rarely does a manga like this not only help you wind down and relax but also share little tips to be happier in general, which is what Aria does. Art: it starts off fairly mundane but only gets better with every volume. Character designs are various degrees of adorable and help you paint a picture of the kind of people that live in the multi-faceted planet of Aqua filled with all kindsof wonders.
Amano's got a real talent to capture beauty of both nature and industry as well as their uncommon harmony. There are a lot of absolutely gorgeous spreads and textless panels focused just on tranquil charm of environment.

Story: while Aria has some overarching plot progression it mostly comes down to three main characters becoming prima undines, the rest are chapter contained stories about finding beauty and happiness in life and in my opinion it more than succeeds in that. Aria is not gonna tell you how to deal with the real world filled with many more flaws, dangers and awful people but everyone will find something to relate here, I'm sure of it.

Funfactor: there isn't a person I wouldn't recommend this to. It's just a great general read, no action or heavy drama, only mellow joy of living.