Rabbits prefer cooler temperatures and their ideal environment would probably be 60-70 degrees. This isn't particularly economical in the summertime in most places but there are other things you can do to help keep them cool. I have granite floor tiles placed in their cage and free roaming area that stay cooler than the rugs and they like to lie on those when it gets hot. I also have a small fan I aim at one of the tiles in their cage so that if they choose to they can sit in front of the fan (and they often do). They were afraid of the fan at first but have grown to appreciate it. Just make sure they can get away from the fan if they want to and that they don't have access to any cords. You can also place tiles or full water bottles in the freezer and then offer them those. Some rabbits will snuggle up next to a frozen water bottle.
Make sure they always have access to unlimited water. They usually drink more from a bowl than from a bottle.
As far as humidity, I run a humidifier in the winter when it's dry for my rabbit who has respiratory issues, but don't regularly use one in the summer.
