Solivagant:
Just to clarify 1 thing and avoid possible misunderstandings - as far as I am aware and have always thought - it isn't necessary to be able to visit ALL the sites in the Hotspot in ONE day - rather EACH of them should be within a day return trip. So 5 sites in a Hotspot COULD mean up to 5 separate day trips (though this is unlikely since several may easily be seen in 1 day). Correct??
That was how I always understood it.
The 'there and back in a day' does cause a few problems though, especially in Europe, as the transport infrastructure is very dense.
I will leave these out from a hotspot but as an example all of these are perfectly visitable in a day trip from London, if you get the flights or Eurostar booking correct, in fact they probably work out cheaper than visiting some of the sites in southern England.
Pretty much
every Belgian site is in range. Though the 4 Lifts and Spieness Flint mines are a slight logisitical problem as they are not as easy to get to by train.
Luxembourg and
Kinderdijk could be squeezed in as well, at a push. In fact it is probably quicker and easier to get to Brussels than it is to most English cites and depending on when you book the tickets, it could even be cheaper.
The sites surrounding
Paris (perhaps Provins is pushing it), and the
Vauban site (Arras)
Amiens are also in range from Lille.
With TGV connections
Reims,
Nancy and perhaps
Lyon though the costs start to go up considerably then.
Then a selection using budget flights:
Berlin, 3 sites, the Housing estate at Falkenburg is right next to Schonfeld airport the 'budget' terminus of Berlin
Bremen, a very easy trip as the airport is very close to the city centre
Volkingen, a bit of a stretch but I did it from Dusseldorf Weeze, but Dortmund is also doable.
Amsterdam +
Beemster and
Defence Line (though flights are never that cheap) The Dutch train system is very efficient from Schipol so a lot of others are viable.
Jelling Mounds, within range of Billund Airport, all in the trip cost me just over £50
Valencia, I did this one in shock at the price of a return train ticket from London. It actually worked out cheaper to go to Valencia than Bath, though I did stay on for longer.
Tarragona, right next to Reus airport
Bru na Boinne, lots of cheap flights to Dublin, courtesy of Mr O'Leary
There are plenty of others when you look.
When I lived in Dublin, it was even easier to do these international day trips as the airport is closer to the city centre.
Sorry winterkjm, but I guess the weather is a little warmer in LA so that can be a consolation.