My wife and I plan to travel to Turkey in late October and will begin in Istanbul and then look to spend 5 nights on the coast. I know it will not necessarily be beach weather, but we are still looking for a very nice hotel with great grounds (pools, spa, gym) and good (doesn't have to be great) restaurants on property or very nearby. Days will be spent exploring the area - ancient sites, coast, hills - but we will also enjoy spending nice downtime at the hotel.

I don't see how we can go to the Turkish coast and not do a day trip to Ephesus, so does that make our decision for us? Or are there some spectacular ruins down south that would allow us to opt for there?

So I welcome recommendation for which location and any specific hotel properties in either area (and the research I've done does show many are closed for the season Nov 1). Looking to pamper ourselves, as I am currently being treated for throat cancer and we are hopeful Turkey will provide a much-deserved celebratory experience,

Also leaning Swisshotel Bosphorus in Istanbul, but are considering Ritz Carlton, Four Seasons, or Cirigan Palace.

Best wishes on your recovery from cancer.
We stayed at the Ritz Carlton in Istanbul and do not recommend it. It is in the middle of many streets making it a traffic circle Stay up the hill in Taksim area, Hotel Intercontinental etc.
Plus, we had terrible service at the Ritz Carlton.

Not too far from Ephesus you can do the sights of Priene, Meletus and Didyma. I highly recommend all three. They are Greek but each unique. We did all in a days drive between Bodrum and Ephesus but then we stayed a few days near Ephesus. Not sure what is all open in November on the coast, but Bodrum does have a Mandarin Oriental and a bit closer to Ephesus there is a Six Senses. Or if you don't really need full services consider renting a villa through AirBnB or similar.

Pergamon is to the north of Izmir while Ephesus is to the south of Izmir, both very worthwhile ancient sites.

In Istanbul, I suggest Park Bosporus, almost at Taksim. All amenities and a umber of restaurants on the roof. Vey close to our home. My wife has a fitness membership there (currently frozen due to the pandemic

or, for something different, The Stay Bosporus. lovely location. on the water

or to be completely different, Hotel Sumahan, on the Anatolian side of Bosporus. Owned by an architect couple, Canadian and Turkish.
The wife was on the board of trustees of the Istanbul Science center Foundation with me.

Staying at Kusadasi will be pracical fo Ephesus and the other smaller antique sites already mentioned, plus my suggestion of Sardis &/or Aphrodisias.

If you wish to see a great deal historic and natural sites, you should rent a car and drive to all. Quite easy. Then pick a small hotel, like the Hilton Garden Inn. If you wish to spend most of your time in the hotel and if lucky on the beach, go for the larger all inclusive resort hotels.
You will have more hotel choices in Bodrum but not good access to historic sites.
Do not forget that you will bel;imited by the short daylight hours.

For the great advice. Looking forward to our visit in September.
Neil

Either Kusadasi for Ephesos etc or something in a completely different (and warmer area):
Antalya wit the impressing antique cities (excavations) of Perge, Side, Aspendos, Phaselis, etc.