CTI feels more like a hotel resort where CSS has more character and rooms are different. The AN at CTI is restricted to the island that has no beach just a pool. Some days closed because the boat back and forth is not running due to choppy seas. They have trading days so if you stay at one you can go over to the other for the day. Lots of folks coming over to CSS but not many the other way.
We have stayed anywhere from 5 nights to 10. I think 8 is perfect as the first day is usually just drinks and dinner with getting there after 2pm. The last day is usually breakfast maybe a swim then off to airport.
Once you get there and get into the vibe you won’t want to leave to go anywhere. There are some excursions that the resort provides. We did a couple years ago but no interest now.
The gym is more than basic but not club like.. cardio, weights and some stretching areas. There are yoga classes and some water aerobics in the mineral pool. You can check and see if there is a guest instructor coming in for your time.
There are some good Facebook groups for the resort that are very informative on the resort as well as the AN aspect.
Try Couples San Souci Fans it’s a closed group but easily accessible