Feeding frozen treats isn't exactly bad for your rabbit as opposed to feeding them something raw and fresh. (In other words, it's not going to harm your rabbit to have something frozen. It just wouldn't be as beneficial as something fresh would be.) Yes, freezing can take away some of the nutrients, but not all of them. If you still want to feed frozen treats, I would suggest fruits instead of veggies. Vegetable, especially leafy greens don't do well being frozen, and in fact you'll just kill them by sticking them in the freezer and you'll end up with a pile of black mush. Berries especially can still keep most of their nutrients when frozen. I don't think it's a bad idea at all to feed frozen treats (in small portions) to your rabbits when it's hot outside.