Neilsons is good - always seems better value than Warners when I've compared (due to more being included). If you can go out of school holiday season and for 2 weeks then you can get some great deals. They have tuition for sailing, windsurfing, tennis, waterskiing, guided cycle rides for all levels, some places do rock climbing too. We've had some great holidays.
Decent evening activities. I don't drink and that hasn't been an issue. Lots of kids of all ages. I've learned to scuba dive. Had sailing lessons but didn't really like it. Kayaking and SUP all available as well as windsurf plus cycling and tennis (lessons, social games etc).
I wouldn't say the clientele is desperately old. Maybe average 35-50.
We went to Ocean Elements in Vassiliki with 3 older teens. you sound a lot like me! I was determined not to enjoy it if I am honest, but I really did. you can do a start sailing or start windsurfing course for half the day and the evening entertainment is not tacky just great fun. its on the same beach as Club Vass which is mainly windsurfing rather than sailing. they also did a days cycle ride, early morning runs that sort of thing. would go back there again.