If you're at CSA the one "must-do" excursion is to walk south on the beach for an afternoon. You'll see all kinds of resorts, restaurants, bars, stores, folks selling stuff, etc. It is really fun, free, and on your own schedule. If you walk too far you can just grab a cab back.
Highly recommend the zip line excursion through Chukka Tours. My husband and I enjoyed it so much in 2007, we're doing it again this April. You can book the excursion through the resort (we were at CN). We did the Black River/Appleton Rum Factory/YS Falls tour in 2007 as well -- while it was fun, it did not compare to the zip line. The canopies were breathtaking.
ATV RIDE!! We were gone 6 out of 7 days when we went. The ATV ride was by far the best! So much fun and you get to see the mountains and we got to visit a little town with a school. It doesn't take long to get there either which is nice. If you don't like that, then the other one we loved was the black river/rum tour/falls. You have to do all 3 together though. You do the rum tour last and it is a longer drive back to the resort. You are gone all day long. So very worth it though.
dnbmccann, do you each get your own ATV or does one couple get an ATV together? We were thinking about doing this in April - sounds like a lot of fun.
As for Dunns River Falls, it was fun to do when we were at CTI (it's included in the activities), but if you're at CN or CSA, that would be a 4 hour drive one way trip...ouch!
Bobby and Lisa- yes, each person gets their own ATV. I had never been on an ATV before but I went and had a blast and they were so easy to ride! We also did the dune buggy's at CSS, those fit 2 people so only one person drove.
Do you want to do fun exciting or fun interesting? Exciting would be the zip line or ATV. Interesting is the Black River with its crocodiles, Appleton Rum Factory with its tastings at the end, YS Falls with its foliage, natural swimming pools, tubing, and scenic falls. Rick's is worth a stop if you have time just to say you were there. Expensive and crowded it does have the cliff divers to watch but be prepared to be pestered for tips from them. You can just get a cab there so don't include it in a tour. You may not want to stay all that long. Last year we hired a driver for the day (check travel sites for suggestions) and visited a few of these all in one day. Things are spread out in the Negril area though. Margaritaville is just a short walk down the beach (turn left) and pretty honky tonk. Only went in once during a walk to see it. The main thing is to not over think or book excursions. Once there you will just want to relax and lay on the beach or on a float with a drink in hand. It is Jamaica mon.
I would suggest the sunset catamaran cruise, sign up for it as soon as you arrive. You get decent rum punch, view of the coastline, slide past Rick's (no tipping necessary for the divers) and a gorgeous sunset from the water. Be advised the cat cruise is part of the A/I package but the staff on the boat are not couples staff and we do tip them $10.00 per couple I think is the norm?