Personally, I would just keep them in the fridge - I keep their greens in the fridge just to help them stay fresh, and I let them warm up a bit in the morning, but the rabbits don't seem to mind the tiny chill.
Putting them in the freezer seems a little dodgy since the veggies could freeze and it'd take even longer to defrost them, not to mention the havoc frozen veg might play on their tummies... I wouldn't risk it! Just slip them in the fridge and they should be fine <3