The Wife and I did Zimbali back in February. As Seveen said, remember to wear sturdy shoes. I wore flip-flops on the farm tour, and wished that I had worn sneakers, at least. There is some moderate uphill hiking. The tour was awesome, nonetheless. And yes, the farm tour was HOT, but the retreat building itself was comfortable and breezy. If you haven't decided yet, I would go for lunch as opposed to dinner. I don't know how the farm tour works during the dinner seating, but obviously it is easier to see things during daylight hours. In addition, I recommend that you use their driver to pick you up at your hotel. Some drivers in Negril don't know how to get to Zimbali as it is not a straightforward route. The price they charge for transportation is reasonable, but they do try to squeeze 4 passengers in the car so you may have to share the taxi with another couple, if there is only 2 people in your party. (Actually, on the ride back to Negril we squeezed a 5th passenger in the car, as our driver agreed to give somebody a lift to a nearby route taxi stop.)
When we went for lunch, fish was the only option for an entree. I guess we will just have to go back again and try the chicken!