Actually, your 19 year old brother is allowed in all the adult nightclubs, shows, etc. He just can't be served alcohol. The teen program (Common Grounds on the WOnder, The Stack on the Magic) has programming from 12:30 pm till 1-2 am daily--parties every night, lots to do, etc. You can just "hang" in the area or participate in the activities.
Deck 9 is the pool deck. You can swim, tan, play ping pong, shuffleboard, eat, etc....there's an internet lounge, adult and teen programming, Broadway style shows, lounge entertainment acts (open to all before 9 pm, adults only after 9).
My teen plans to sleep till noon every day and party all night, although that would mean missing breakfast and it MIGHT be necessary to get up for breakfast.
You'll have lots to do!