Your included excursion to Dunn's River Falls is just the excursion to the falls. You do climb up with a guide. There is no zip lining tour with it. You can book zip line tours at the tour desk at CTI once you arrive.
We did the excursion from CSS in May and it was just the trip to the falls, think it took about 2 1/2 - 3 hrs total. Couples provides the bus to the falls and everyone gets a ticket from the resort for entrance and that includes your guide(s) up the falls. The guides absolutely expect a tip at the end and that was ok w/us because if you take a waterproof camera they will take 15-20 photos of you w/your own camera all the way up the falls.