Consider staying in Playa del Carmen or Akumal to get away from rowdiness of Cancun and closer to sightseeing in Tulum.
I like the Dreams resorts best for families in Cancun. Best beach-Dreams Tulum, but the others are all nice as well. How old is a young adult?
Anywhere south of Cancun (easily driveable and/or reachable by hired transport) is awesome. Playa del Carmen is nice with enough of a lively (but increasingly party) scene. Go further south towards Tulum and that's where things get far more relaxing with nice beaches, not too many crowds and still enough laid-back but great restaurants.
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Not looking for a really busy, active vacation. Maybe some sight-seeing, but a lot of lounging in the ocean as well.