Based on what you list liking and wanting more to do for the 24 year old, I also recomend
Royal Caribbean.
If you have any status with Celebrity, yo ucan call and have it matched on RCCI---they are owned by the same company. What you get as perks is different between the lines--you can easily look it up though. If you happen to be Elite on Celebrity (not sure if you have sailed that much), you'd be diamond on RCCI and get three drink vouchers each to use in any bar in the evenings (usually 6:00-8:30, though it varies a bit by ship) as well as other perks.
Sometimes there are special for dirnk packages, etc--though it is less likely in high volumes time frames like a spring break cruise that you are looking at. Or, they might offer 20% off online if you prebuy it once booking the cruise (this has been teh case for our last couple of cruises).
RCI also allows you to bring on two bottles of wine per cabin---in case not everyone wants a package but someone might still want a bit of wine on the balcony or something.
There is also usually a discounted speicality restraunt package you can buy online before you sail---works out to about 35% off with 3 or 5 nights out (usually)
Cruise critic is by far and a way your best source for information about lines other than
DCL