Japan and the US should trade islands - I propose a small island off the coast of Seattle, and a similar-sized one off the coast of Japan. They don't need to have any natural resources, or good climate, or soil.
The islands sovereignty and legal systems would be traded. Residents would be offered to be bought out, and paid to leave, or would be offered citizenship in the new country, or could accept a generous relocation package within the original country, plus a large donation to a cause or issue they cared about. Care would have to be taken to not take people's homeland away from them - it'd be better to get an island that had been a former national park, or a place with bad weather.
On the official day of the trade, the island off the coast of seattle would officially become part of Japan, and its counterpart in the Japanese archipelago would officially become the westernmost point in the US.
If you look at Hong Kong, it's one chunk of land among hundreds of similar ones along the twisting Chinese coast. Similar small peninsulas in the mainland have some farmers, some mines, but are pretty much undeveloped. They provide a home and a few resources for the locals, but compare that to HK, which has a sophisticated and different culture people, and its own traditions of art, music, movies, food, and language. It's also rich, and has a good government - and it has been an interesting thing for the mainland gov't and people in Shenzhen to see, right next door - showing them other options for organizing society. The process of taking a random piece of land and making it into HK was a good one. Would you rather have 5 million more Chinese farmers, or would you rather have had their grandparents take part in a grand experiment, and today be part of a new, interesting country? Even from an honest CCP perspective, they probably wouldn't go back in time and prevent the formation of HK - its presence helped them over the years. If the other concessions had stayed part of their originating countries, it'd have been even better - Tianjin, Qingdao, etc. And I'm not opposed to giving China islands in Europe, either. We need interlocking, overlapping, easy ways to expose different styles of life and government to each other.
I also wish Venezuela were closer to the US, to illustrate their failure more clearly to Americans (and to help them)
There are lots of other fairly undeveloped pieces of land along the coasts of many countries. So I think they should be traded. There should be enclaves from 10 different countries along China's coast - and there should be little bits of China spread out across the world. China should do it with Africa, the US, everyone. Russia and Austrlia are huge, and their government is limited in what it could do - but think of what the US could do with a tiny piece of northern Australia - or what Australia could do in return with a now-unused piece of western Canada.
To be able to jump on a boat off in San Francisco and enter Japan or China in just an hour - think of the movie opportunities. Plus, think of the real estate prices! The extradition treaties! It'd be like a super intense Chinatown on treasure island, that is literally China.