I would say anywhere between 76-86F is fine for most reef animals. An acceptable daily swing, on the other hand, should be somewhere in the range of + or - 6F or less. If your daily swing is a few degrees more every now and then, it's not a huge deal, but some amount of stability is necessary to keep your animals from becoming too stressed.
If you're looking for simple cooling options, the easiest to implement is blowing some type of fan across the surface of the water. This helps to increase evaporation, which can cool your tank water by up to 5 degrees or more. I put my fan on the same timer as my lights, that way during the day when the tank gets the warmest, the fan is able to keep things cooler. Then at night, when the fan would only serve to work against the heater, it is off and the heater is able to work without being impeded.