Not sure when this will get posted, but at the time of submitting this there was only one reply and that person would also like a little info, so I'm posting this to bump it back to the top in case it has been missed.
Sign up is necessary for water activities such as snorkeling, catamaran cruise, glass bottom boat, and scuba. The times we have been at CN (May-June timeframe), there was never a need to do this upon first arrival. With the exception of the catamaran cruise, you can typically sign up for the activities the same day. You definitely need to sign up in advance for the catamaran cruise though. Other times of year may be more crowded so maybe others can respond regarding sign up.
Hobie cats and kayaks can be taken out any time during hours of operation, without sign up.
Shuttle to Margaritaville
Shopping excursions to Downtown Negril
Watersports & Scuba
Glass-bottom Boat Tour
Catamaran Cruise (sign up morning of and a MUST DO)
Unlimited Golf (transfers and green fees included. Caddy mandatory at a nominal charge)
Negril Craft Market
Royal Palm Reserve
Appleton Rum Estate Tour
Black River Safari
Office of Nature (short walk from CN)
Negril Point Lighthouse