We did not go on the Grand, but were on her sister ship, the Golden last month. I have a 13 and 16 year old. They both loved the cruise.
First, there are no water slides on the Golden so my assumption is there are none on the Grand, but I can't say for sure. However, the pools are fresh water which pleased my kids. Most kids swam in the two main pools. Mini golf and paddle tennis are available and my kids enjoyed both.
My 13 year old boy loved the teen program, especially the ping pong tournament. He was always busy with an activity they had planned and skipped dinner with us a few nights so he could make an event. He was so tired at the end of the trip that he pretty much collapsed at the hotel and slept through the afternoon and night. I was very pleased with the teen staff on the Golden. My 16 year old did a few events, but he wanted to read and go to the shows.
My kids have been on some really great vacations, Disney World, Hawaii, Alaska...., but after the Princess cruise this is what they want to do again. We suggested Maui for next year and they said no they would rather go on another cruise. I hope it turns out as well for you.
I had the same worries you had (about the kids needing something in particular) I was fixated on having a game room because that is what they have enjoyed at other hotels in the evening. They spent about 15 minutes playing video games the entire cruise and instead had lots of fun in other ways.
Have Fun
