Go to the Grotto! It is a mineral spring, so it's cold and very natural. I swear it helped my sunburn on the backs on my legs feel better. It is on the opposite side of the resort as SSB, closer to the lobby. I would recommend on arrival either take a tour, or walk down the path towards G Block and Charlie's Spa. Here you can check out the waterfall, and the lush foliage. Then stop by the Mineral Pool for a fruit smoothie, check out the Grotto and trek over towards the beach. Yes, this is the step-enhanced tour, but you get to see almost the whole place on this loop. By this time, your glass is empty, so check out the beach grill and grab a drink of the day. I have always enjoyed the catamaran rides, so if you are so inclined, enjoy or head right around the corner to SSB. The lake is beautiful with the birds and trees, so it's a nice photo op. Over at SSB, you can get lunch at the grill or hop back over to Palazzina to get out of the sun for the buffet lunch. Before you leave, there are lovely hammocks at the crest of the hill by D Block that overlook the cove - another great photo op and place to enjoy each other.
Be sure to check out the mineral pool and grotto area. The view from the pool is spectacular and the Smoothie bar is located here. There is also a jacuzzi tucked up on the hill that you can access from the mineral pool.
Walk around the pond that is located in the back of blocks B/A. This is where the tennis courts are located. The pond is lovely and there are all sorts of fish and wildlife.
I say definitely go for a walk and explore the resort. There are little paths from the lobby down to the mineral pool, where you'll find hammocks and the hidden hot tub. Below the mineral pool you can go down past the sauna and massage beds to the little beach. If you go out past the mineral pool to the right you can check out the spa massage hut area. In the other direction, (left of the mineral pool) is the mineral grotto, random seating and the gazebo. If you walk past the beach and pool, you can walk all around the pond.
Taking kayaks out and going right off the beach and around past the gazebo, little beach, mineral pool and spa area is also a good way to see the resort while enjoying being on the water.
If you do this full tour, it'll give you a real feel for the resort and you'll likely want to return and spend more time there
I suggest having a bite to eat right down on the beach, since you can't exactly do that at CTI. We really liked being able to wander just a few feet from our chairs to the tables and sit and eat right there (unless of course you're at Sunset beach)
You won't be able to do sunset at the au natural beach as it starts at 5pm and trading places ends at 4pm.
I still don't understand why it can't go until at least 6pm. Other resorts offer exchange privledges including eating dinner and have shuttles running on a regular schedule. I know the dreaded S resorts do this so why can't Couples?
Definitely explore the grounds, as others have suggested, but make sure to take a walk by (or around) the pond. You can feed the funny-looking ducks (I think they're Muscovy ducks, but we call them turduckens or Hershel (after a very strange and awful movie about a guy who turns into a half-turkey-half-man, but I digress) and often see other types of birds, turtles, etc.