Interesting thread all round:

Maybe a water bed with ice in it????? Condensation could be
contained by the liner? One might wake rather damp in the morning
though?

I've always claimed "that the climate here is similar but colder
than the UK" But even at the same latitude as Spain, on this side
of the Atlantic though, it is on average colder and being
adjacent to the North Atlantic, well moderated so that while it
doesn't get as cold as central North America it can be cold and
foggy in the summer!

Well; yesterday we finally got some warm weather here (eastern
Canada) about time too, mid July!

It got up to a somewhat humid 26 Celsius which we find hot. I can
just hear our Florida friends snicker and say "Call that hot"?
That's their winter temperature! Last night it cooled right down,
the advantage of being on the coast so that even with the annual
crop of icebergs all drifted away to melt in the Gulf Stream it
was comfortable for sleeping. Later I had to close the window it
got quite chilly.

Two things;
Vent the roof;
1) Our roof is already well vented but today I must remember to
get the ladder and open the 'big' vent in the gable end of the
roof to vent hot air (sometimes we also install a temporary fan
mounted on a piece of plywood) if this weather continues. Must
also put in the temporary 'fly screen' we had a wasps nest in
there one year!
Use basement (if available);
2) Our approx. 1500 sq. foot by 8 foot high, in ground concrete
basement stays cool at about ground temperature, plus other
losses and gains; roughly at 55 deg. F in winter and
approximately 60 deg. F in summer. So it is possible to close up
the house tight early on a hot day and circulate the upstairs
warm and basement cool air with temporary fans to attain some
degree of comfort. This has worked for several hours until say
around 4.00 PM. Between then and sunset though it gets rather
warm and one would like to be able to go in and out of an
evening. One problem though can be the basement gets damp with
warm humid upstairs and outside air getting to dew point down
there; so some days later, on a dry day, one 'airs out' the
basement thoroughly. We also use a small dehumidifier in a 9 foot
by 11 foot electronics workshop down there if required.
When we lived in Liverpool, big old house turned into flats, a
basement work/storage area was available. On an occasional hot
night it was just as well to go down there into the cool and do
some d-i-y in the workshop, rather than toss and turn in the heat
upstairs!
If necessary the basement is a great place to sleep!