I was browsing and found a thread which mentioned the Delfini Hotel in Santorini (and, no, I can't imagine what that thing is on the floor, or the thing in the sink) but I was really intrigued by the cave hotels. It got me dreaming - what other really cool hotels/apartments are there? I would love to include something like that in future trips, and would even plan a trip around a great place. I know about the ice hotel, but what else is there?

the hotels that I'm interested in can be anywhere, I'm looking for unique and comfortable.

Google underwater hotels. There's a new one in Dubai.

Propeller Island City Lodge in Berlin. Each room is an art installation, with a different theme. You can sleep in the upside down room (all furniture nailed to the ceiling; you open up a compartment in the floor to find your bed), a cell, the floating bed room, etc. Even if you don't ever stay there (but you should because it's surprisingly affordable and conveniently located), it's worth having a look at the rooms. Go to www.propeller-island.com

An old mud & straw kasbah in Skoura, Morocco called Ait Ben Moro. Roof looks out on the palmarie, the Atlas Mountains and the Valley of 1000 Kasbahs.

http://www.ilove-marrakesh.com/kasba.../index_en.html

It may not be "5 star", but it's not roughing it.

How about a crane, a light house or a lifeboat? Or a watertower? All available in the Netherlands. The crane, lighthouse and lifeboat are all one room hotels. Not cheap but a unique experience!

http://www.vuurtoren-harlingen.nl/
http://www.dekoperenhoogte.nl/

http://www.distinctlydifferent.co.uk/

http://www.accommodationbritain.co.u...s/Unusual.html

any of these would be really fun - thanks for the ideas! I will be researching them.

any more out there?

Since you mentioned being intrigued by cave hotels, the ones to be found in Cappadocia in Turkey are amazing.
I stayed at the Kelebek Pensione in Goreme (its an Inn spread out over two hills and several streets. Theres a hotel part to it too) and throughly enjoyed my 3 days there. Beautiful, uniquely furnished rooms and living space and if requested, a basket of kindling and firewood will be left by your door on chilly nights for the waist-high, in-wall fireplace.

Terrific views and good food too.

Take a look : http://www.kelebekhotel.com/classic/classic.php