McDonald's Happy Meal Watch" - really it is a very rudimentary "fitness tracker." Recalled for causing skin burns and rashes.
(Above picture is not an xray, but is the tracker backlit by a strong, white LED light.)
So here is "STEP-iT, a very cheap and rudimentary "fitness tracker" that was included as the toy in McDonald's Happy Meals in the USA, and maybe Canada. They were released sometime in the summer of 2016, and then recalled after they caused rashes and burns. It's all over Google if you want to search it up. I can't imagine how inexpensive these were if they were given as the free "toy" in a box with a burger, and greasy fries. It's not really at fitness tracker. All it does is count steps. I guess you can't really expect much for a free toy.
Here is a view of the back. Mine is obviously a blue watch, but there were other colors. I saw a Youtube video of this dude trying to open it up. It was not meant to be opened, but he did manage to find a seam and break it open to get at the parts. 

There is no metal touching your skin. The screws and other parts are all behind the back. It seems like most commonly it is metal that causes irritation and rashes. Seems strange that plastic would, but certainly any material could potentially be an irritant. As far as thermal burns, the "burns" or hot feelings that people experienced. This also seems hard to explain. It's got a low voltage battery typical for such cheap devices, and I can't understand how this device can generate enough heat to burn anyone.
Instructions, above. It looks like maybe there are three working buttons, but the only thing you can push is the on/off button in the lower center. This resets the "steps." I say "steps" because it is pretty inaccurate. Notice it encourages people to challenge their friends. Tell your friends to go eat a 975 calorie meal, get the watch, and then challenge you to a brisk walk.
The device may not be exactly square, but it's pretty close to it. Here you also get a partial view of the transparent blue strap.
In the toy packet, there was a round card, which had some instructions on how you can download "McPlay," scan your toy and play games, but I never did that. I doubt it's supported at this point.
Happy Meal, in Japan. "Happī Setto," or "Happy Set" in Katakana. 

Happy Meal, in Spanish-speaking countries. 'Cajita Feliz" means "Small Happy Box."

Happy Meal was invented in 1979, but the genesis of the idea comes from Yolanda Fernández de Cofiño, in Guatemala, who invented "Menu Ronald" in the early 1970s. Wiki has the whole story.
Part of the strap shown. Those two raised bits slot into these openings on the other side.
The step counter is terrible. The accelerometer must be very cheap. When you move the watch once, it might suddenly count like 39 steps! Seems very hyper-responsive. Again, what you can you expect? It's a free toy in a box of fast food.

The above pic shows a close up. You can partially see the reset button. Here I have walked 2697 steps. Notice the lower case i, in STEP-iT. After the iPod, everybody had to put in that lower case i, to be cool. Everything was the little i.
Here is the other side of that round card I showed above. It features STEP-iT, and a smiling Happy Meal box. It also shows people engaging in running, cycling, kicking and throwing various balls, as part of the active lifestyle.
In addition to my blue one, where were five other colors. Notice that half have the configuration with an LCD panel, but three do not. I'm not sure how the other version works, what exactly it tells you.
/end
Here is Yolanda receiving an award from Ronald McDonald.
Here she is receiving another award. Both awards appear to be statues of Ronald McDonald.
One final look at this Happy Meal "STEP-iT" tracker device that was recalled because it was felt to be dangerous. Rash, overheating etc. As would be expected for a free electronica device, it's pretty cheap, and works only marginally. The styling looks more like a toy than a high-grade device. Battery appears to be irreplaceable. I never actually wore it. But it's a pretty nice piece of history.

I hope you will like it. 

Thanks for reading.

Alan

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Website: Alan's Vintage Watches.