DD loved The Stack. There is a loose programming schedule, but it is like 3 things per day. There is a themed party each night--basically, this is music and hanging out. Dance if you want to. There are video games, video cameras where they film themselves, dancing (no, you don't have to), a long list of "possible activities, ask CM for supplies" that includes games, crafts, etc. They also have times on the sports deck, one session in the fitness center, etc.
Yes, go to the 411 meeting on the first night--they will hear all the activities, but also all the rules. One of the security guys talks with them, etc. And they meet each other and have fun.
They have scavenger hunts, boys vs girls activities, etc. They are well supervised. DD has sailed 3 times as a teen. We've only had a problem once, and that was a 16 year old "hitting on" my 13....and accusing her of racism when she wasn't interested. The staff handled it very well, and there were no problems as soon as it was reported to them.