Hotel: Andronis Arcadia, Santorini Greece
Room Booked: Sunset Suite
Rate: Around $700/night
Early Check-in: Not needed
Late Checkout: Yes, and used without issue
Upgrade: Yes, to Natura Suite. Slight communication gap as we had already been proactively upgraded but initially the agent handling our check-in didn't know this or didn't explain it correctly (or there was a language barrier issue), but was fine in the end. There seem to be quite a few Natura suites so I'd imagine this is a commonly successful upgrade, although Santorini is also quieter than usual this summer due to worries about the earthquakes earlier this year.
Resort Credit: Marketed on the FHR site as USD150, honored at the hotel as EUR150, which I did not complain about. Used against a spa experience though could also be applied against F&B. (You can also charge to your room from the restaurants at the other Andronis properties in Santorini, but I did not test if the credit could be used against those charges. We did though have an excellent tasting menu meal at Lycabettus at one of the Andronis properties in Oia, and I'd highly recommend it.)
Breakfast: Appears to be included with all rates here, so not really an FHR-specific benefit, but good nonetheless. Served in the restaurant until 11am and offering your choice of made-to-order dishes from an extensive menu, with everyone encouraged to order as much as they'd like. There was also the offer to take breakfast by room service for a EUR20 supplement per person.
Prepaid/Paid at Hotel: Prepaid
Loyalty Points/Benefits Received?: Did not attempt -- participates in GHA Discovery
Comment: I'll share more on the Santorini thread, but overall I enjoyed this hotel a lot and I would definitely recommend, though it is not the classic cave hotel experience that some may have their hearts set on for a Santorini trip. But if you want to be very close to Oia while having the benefit of a typical modern resort (and not having to put up with the quirks of typical Greek hotels, like tiny rooms with awkward layouts, and small pools / minimal other resort amenities), this is a great option.