Interesting niche subset which is closely linked to the WHS hotspots - others that come to mind and which I visited in 1 day :
several WHS around Brussels, several WHS around Beijing, several WHS all over Italy, several WHS all over Germany and France, several WHS around Delhi and Agra, several WHS in Spain and Portugal, any WHS + Struve in the Baltics, the desert WHS of Jordan, several Mayan WHS in Yucatan, Mexico, practically all/most WHS in Malta and Cyprus, several WHS on the mainland of Greece, Statue of Liberty + Independence Hall can be done in a day (I visited on separate US visits), most WHS near Copenhagen, Denmark, most WHS near Stockholm, Sweden, Azerbaigan's WHS, most WHS near Vienna, Austria (including Graz), several WHS in Australia (ex. Sydney Opera House + Hyde Barracks, Royal Exhibition Building + Tasmania, Sydney + Lord Howe Island, Great Barrier Reef + Wet Tropics of Queensland and/or Fraser Island, etc.), Albania's WHS + Corfu, several Balkan WHS (Dubrovnik, Mostar and Stecci, Kotor + Dubrovnik, Dubrovnik + Stari Grad, etc.), several WHS in Palestine/Israel, several WHS in Iran, several WHS in Turkey, several in Southern Poland, several in Czechia, several in Slovakia + Hungary.
Most I'd tick on the same day but then allow more time in one of them or revisit. Actually the mentioned Guiness World Record is quite easy to beat if you were to use a rental car/private car anywhere in Europe and simply want to be in more than 12 WHS in 1 day - but that would be far from actually visiting them!