Depends on their age - I went to Carvoeiro a few times as a teenager and enjoyed the sunbathing, nearby waterpark, shopping, horse riding and of course the bars at night - which will serve under 18s!
Hope that helps.
Euchre, crazy 8's, and gin (cards) are still favourites in our family. There's lots of potential in exploring beaches at night - but take care on the cliffs!!
beaches, pools, tennis, golf, water parks, horse riding, boat trips, fishing, volleyball, football, meting loads of other teenagers, bars, nightclubs.... mind you often depends on resort chosen. I think teenagers for example love Rocha Brava.