Hi!
I traveled on Holland America too! You have to join the teen club first before you can be part of the teen activities/meetings. In the lounge, there are set meetings, games and parties on certain days. I remember there was an Apples to Apples tournament, a Welcome party, a relaxing "free choice" activity and a farewell party (and a lot more...). There are about 5 events going on up there on each day. Also, they have an arcade around too.
At first, there doesn't seem like a lot of teens are on board, but once you see the teen lounge, there are a lot more teens there. You will tend to find less teens onboard Holland America, but the cruise itself is great.
You will really have a lot of fun on the cruise. There's always something to do and a lot to eat (make sure to try as many different foods as you can!). Your waiters (in the dining room) will remember your name and so will your cabin steward. I wish I was going on a cruise this summer too!
Have fun!
