You can take a 2 liter bottle of soda with water, really any container, freeze it and put it in your rabbits area. I usually put one in front of a fan and they lay in front of in to cool off and a couple in random resting places so they can sleep comfortably during the day. Its going to be 100+ for the next few days in SoCal, so I froze bottles last night to use during the day today.
All the suggestion make by other users works great too. Stone tiles, ice in water bowls and wetting ears will also help keep your bunny cool. If you use a water bottle for your bunnies, try freezing half a water bottle and adding some water to it before hanging it up for your bunnies. The ice can melt through out day while you are not home.