I was planning a trip to Sealand a few years ago; apparently you can negotiate trips with the 'Prince of Sealand', and even acquire a Sealand passport for yours

I was planning a trip to Sealand a few years ago; apparently you can negotiate trips with the 'Prince of Sealand', and even acquire a Sealand passport for yourself. But then it turned out the fort is in somewhat bad shape and getting on and off can be a bit hazardous, so we ultimately decided against going :)