Colonia Sant Jordi does not get a lot of British/Irish visitors. The tourists there are mostly Germans, and then French and Italians. Some people quite like this as it means that you can't overhear the next table (and vice versa), but it might make it a little harder for DC6 to make friends, if that matters, and when you go into a restaurant you are quite likely to be greeted with "Guten Tag".
There is an aquarium in the town and a visit to the nearby salt works is worth doing. As PP noted, the famous Es Trenc beach starts at Colonia Sant Jordi and goes north, but you may see some bare bums, and if you wander off into the dunes you may see a bit more than that. 😳 There aren't really designated nudist beaches in Mallorca, as it's legal to be naked on almost any beach, but some are more "textile-free" than others by local custom. A 10 minute walk to the south is a "hidden gem" beach called Es Carbó, which is not known for bare bums, but be prepared if that's an issue for you.
The star attraction is a boat ride over to the island of Cabrera. This costs between €40 and €75 per person depending on length of stay and what else is included. It's a great place for hiking. The island is officially uninhabited but there is a bar with tapas, a ranger station, and a police presence — see if you can spot the radar on top of the island which tries to pick up the boats coming from North Africa before they are dashed onto the rocks. The memorial to the French soldiers is also quite an interesting bit of history. Some of the boats stop off at a cave for you to swim on the way back.
Just on the edge of the town is a sports training centre called BEST. It has an Olympic-sized swimming pool, and swimmers and other athletes from all over Europe go there in the winter. I'm not sure if they do day passes or visits but it's on my list to try next time I'm in the area.
The nearest waterpark is at El Arenal, about 30 minutes drive away towards Palma.