I saw a video a bit like this www.tiktok.com/@games4two/video/7296662976726109486?lang=en advertising this silly but slightly addictive game. (It was a different video that I can't find now.)
Basically, you have to place all your magnets in the circle. If two or more clump together you have to pick them all up. The winner is the first player to get rid of all of their magnets.
In the original video with the ad it seemed too expensive, but I found it at AliExpress for just a few pounds (I'm looking at my order right now and it seems they call it "Polished Black Lodestone Magnet Multi-functional Magnetic Block Relieve Stress Entertainment Magnetic therapy Fridge Magnets" 🤣).
It's suitable for any age of child who can be trusted not to swallow the magnets, but adults can play too. You could make a bigger version by combining two sets (all of the sellers seem to put slightly different numbers of magnets in the package) and/or playing with the length of the circle.
It's easily portable (fits into an A6 size envelope) and can be played on any smooth surface, such as an airport floor while waiting too long for the boarding announcement.
I have no connection with any of the manufacturers or resellers other than as an admirer of this very clever idea!