My son is 15 and we have done:
Bahamas - few days at Atlantis with the water park, entertainment then onto Great Exuma for lots of activities such as swimming with pigs, hiring our own speedboat (which he got to drive), deep sea fishing etc. Very friendly place and made loads of friends.
Costa Rica - Caribbean coast, again super friendly loads to see and do, amazing wildlife, chilled beach vibe.
Dubai - don’t rate it as a country and wouldn’t return but again lots to do and he enjoyed it.
Bologna, Italy - we visited the Ferrari museum, went on a good tour, visit some lakes etc.
Skiing - always a winner!
Next holiday will do scuba diving and get our licences.
I always look for any holiday that has activities, adventures. Either as part of the holiday such as we’ve done a surf camp holiday or like Bahamas I book lots of day activities.
I’ve also done Orlando theme parks with him and tbh it’s the only holiday we’ve ever had that both of us felt a bit lonely on!
The reason being is everyone at the hotels are out and about everyday visiting different parks and so you never get a chance to meet anyone or getting chatting to people which I’ve found really important when travelling solo with a child.
Oh one other trip he really wants to do is go visit wild America, see buffalo / the plains, maybe a ranch etc.
Hope some of those ideas help 