So, I was wondering if sf authors have any useful ideas for devices that are either wearable or skin-implantable, and provide some sort of visible display outside the body.

Most of the ideas tend to combine a display with information that is of vital interest to the person and to others. For example, consider the palmflower from the 1967 novel Logan's Run by William Nolan and GC Johnson (the picture is from the 1976 movie of the same name starring Michael York):

"Show me your hand, Logan," said the psyc doctor.
Logan obeyed.
"Do you know why you have this?" he said, tapping the palmflower with an index finger.
"To tell my age," said Logan.
"And how old are you?"
"I'm six."
And what happens when you are seven?"
"It goes to blue... and I leave the nursery."

A similar idea is the cardioplate from Harlan Ellison's 1965 story 'Repent Harlequin' Said the Ticktockman. This idea was used in the 2011 film In Time; their usage is shown in the picture below.

Another example of a device that shows others information about you is the ER - emotional register, a small disk worn in the center of the forehead that shows the emotional state of the person wearing it. The ER is from the 1961 story The Primal Urge by Brian Aldiss.

One feature only was definitely not, as yet all events, ordinary: the shining circle. Three and a half centimetres in diameter, permanently fixed in the centre of his forehead. Made of a metal resembling stainless steel, its surface was slightly convex, so that it gave a vague and distorted image of the world before it.

One possibly more functional idea is the Handwriter from John Varley's 1992 novel Steel Beach (the characters appear in the related readout skin):

Call me old-fashioned. I'm the only reporter I know who still uses his handwriter except to take notes…I snapped the fingers of my left hand…Three rows of four colored dots appeared on the heel of my left hand. By pressing the dots in different combinations with my fingertips I was able to write the story in shorthand...

I also thought of the decorative implants from Samuel R. Delany's 1968 Nebula award-winning novel Babel 17:

"It's listed in your catalog as 5463," the Customs Officer declared. "I want it there." He clapped his left hand to his right shoulder.

The surgeon returned ... with a tray full of fragments. The only recognizable one was the front half of a miniature dragon with jeweled eyes, glittering sc ales, and opalescent wings: it was less than two inches long.

"When he's connected up to your nervous system, you'll be able to make him whistle, hiss, roar, flap his wings and spit sparks..."

Perhaps readers have other ideas?