There is an arcade (games for purchase).
Common Grounds is a place to meet other kids and they facilitate that there. They organize the teen excursions (snorkeling etc.). They organize things like scavenger hunts, basketball games etc. Kids seem to make their own small group of friends to hang out with them. They go to the movies or other shipboard activities together outside of CG on their own.
My daughter has been on three different cruise lines. Common Grounds was her favorite teen club by far. She said they set the rules from the first day and stood by them, unlike the other cruises she's experienced.
She is uncomfortable around kids that make her feel uncomfortable. She picks her friends well and abides by my curfews.
I'm not saying everyone's kid is an angel but I will say that generally "birds of a feather flock together". The troublemakers hang out with the troublemakers and the good kids seem to hang together too.
These DIS board discussions amuse me. DIS cruisers would die if they saw what went on on different cruiselines (we were stepping over bodies in the hallway on
RCCL and drunk kids were running up and down the hallway on Carnival when were were on that). It didn't effect my kids even then though except to give them something to talk about. Shipboard gossip flies fast.
Your relationship with your teen is going to be exactly the same at sea as it is on land. If they say "no" on land they are not going to change at sea. If you have a good abstaining kid you know who you are and if you're kid is a smoker/drinker/drugtaker you know who you are too.
IMO, Disney does the very best job that they can possibly do without having Hook out there making the few bad kids there are walk the plank.
One more thing, my daughter says the caliber of the kids on Disney is higher and she attributes that to the price of the tickets (say compared to Carnival). OK, maybe there's some elitism or snobbism in there...I'm just repeating what she said.