A slightly niche subset I know but last night I got to add to my tally of visiting two sites in the same day which were inscribed in the 1970's. Namely Plitvice Lakes and Split. I think I have done it five times now; Krakow and Wieliczka Salt Mine, Krakow and Auschwitz, Tunis and Carthage, Chartres and Versailles.

I'm pretty sure some people will have a higher count than that as there are 56 sites to play with but I thought I would post it to show how quirkily obsessive I can be and also wonder if anyone else has similar niche collections.

World record of World Heritage sites visited in one day:
http://www.guinnessworldrecords.com/world-records/373467-most-unesco-world-heritage-s ite-visits-in-24-hours

Yes there were a lot of "adjacent" sites inscribed in those first 2 years (78/9) - relatively few countries were active and they took the opportunity
Of the 56 -
I am pretty sure we did Ivanovo and Madara rider on the same day - but it was so long ago I can't remember for certain!
Many people will have done Islamic Cairo and the Pyramids -as have we
We did 3 - Kotor/ Dubrovnik/ Split in a day earlier this year. OK - Dubrovnik was a revisit and it was so crowded we hardly spent any time there and we only arrived late in Split.
Quito and Galapagos will be common for people flying between the Capital and the islands
Similarly many people stay in Antigua Guatemala as it is convenient for the Guatemala City airport and a flight to Tikal (but haven't done so personally)

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!

A Guinness record which is "there for the taking" by anyone who could be bothered I would have thought.
It doesn't make clear what constitutes a "Visit" nor whether there are any transport restrictions. As they used taxis I would have thought that anything would go and a visit should include seeing from the outside or at most "going" inside the inscribed boundary?
My choice would be the Amiens -Amsterdam axis - The road distance from Amiens to Amsterdam via Brussels is 434kms - 4hrs 30 mins direct.
With a few side trips and careful choice from among the numerous Mining sites, Belfries and Beguinages it would seem that up to 21 WHS are available just on that route through Fr/Bel/NL. Bruges is the only big diversion but the road is fast and if you have 24 hours it should be easy
Amiens Cath
Vauban (Arras)
Fr Mining
Bel Mining
Belfries
Beguinages
Spiennes (don't bother with a descent)
4 Lifts
Tournai Cath
Stoclet (would have to put one's hand inside the railing!!)
Grand Place
Horta Houses (into the entrance!)
Prim Beech Forests (Soignes nr Brussels!)
Bruges
Le Corbus (Antwerp)
Plantin Moretus
Kinderdyk
Van Nellie
Rietveld Schroeder
Amst Ring
Amsterdam

Yeah I think that world record could be beaten reasonably easily just on public transport (I'm some people may already have thoughts on the route). But fair play to those guys for giving it a good go and getting an official world record. I think my record is probably 6 in one day in and around Brussels, but I never really tried to set a record.

Though I still claim to hold the unofficial world record for traveling between four on foot :) I keep promising myself I will break it soon.

Though interestingly I just listened to a Planet Money podcast on the new business model of Guinness World Records, well worth a listen if you are interested in that sort of thing, as is Planet Money in general.

I sat down and figured I could "visit" between 19-23 sites in 24 hours pending on traffic.

For me, it's 3 in Iran (Tchoga Zambil, Susa, Shushtar)

The only time I've done three in one day was in Portugal: Monastery of Alcobaca, Monastery of Batalha and Convent of Christ in Tomar are all pretty close together.

If you want to split hairs I could technically claim five for the day, since we drove up from Lisbon and ended the day in Coimbra!

jonathanfr
That does not look like real fun for me.

Hm, the most meaningful multiple-WHS (even if not of the 70's) day that I can remember was in Jordan: an extremely long day starting at the airport at around 5 in the morning, then waiting for our guide and driver, and then off to Qusair Amra (1985, 1) with a nice visit in the fresh morning hours; then lunch at Umm ar-Rasas (2004, 2) after the visit, after which - and here we had already began rushing - a longer haul southwards stopping at Karak's... krak and on again to Wadi Ramm (2011, 3). We got there only at 9 in the evening and were fairly sleep-deprived, but a bumpy ride in the cool night air and roasted chicken at the camp woke us up!

But as niche collections go, I think I don't have any... the closest I come to is probably the list of WHSs (and also not WHSs) that appear as wonders in Age of Empires II: when I get to one, I am particularly satisfied! :-D

And now that I look at the List, I have visited very few of the "originals" of the 70's, 'tis a thing I have to improve, methinks