I spent the day with some friends at Maho Beach on the Dutch side. This is the beach just at the edge of Princess Juliana International Airport. So we saw many planes land while we were on the beach. I would recommend it just for the planes! However, the beach is nice, the sand is fine and not hot on your feet even though the temperature was hot, about 30 Celsius. The water was blue and, clear and warm. This is a popular and busy beach, so if you want quiter beach, you may want to go elsewhere. Other than that, I just spent some time around the docks and visited some stores and vendors. I had my first coconut smoothie there and I would recommend it to anyone!
I did a walk around Old San Juan and San Juan myself. I picked up a map and took my time. I had a coffee at a little outdoor cafe in the heart of Old San Juan, visited Don Collins Cigar Shop, chatted with locals etc. In short I did not take the tourist excursions. It was a nice fun and relaxing day.
This was a private island (owned by Norwegian Cruise Lines) that we were on. The beach and waters were amazing! I practically stayed in the water for as long as I could! We had to come ashore on Tenders (smaller boats) as there was no place for the ship to dock. Just walk a bit further away from where the Tenders drop you off and you will find a enough beach to make your own for the day. You can stay with crowds or go further to be by yourself. There are cabanas and tents to rent if you want some shade. They setup a barbecue for lunch which was good. Though I wish it had stayed open longer. Having said that I understand they needed to bring all the food, drinks, equipment etc. from the ship to the island and then take it all back.