There are two things I might consider. First, I find it amazing that really cold water does not freeze as quickly as warmer water. This might be something that you could do an experiment on.
The hypothesis would have to reflect that water temperature is not necessary to the rate of water freezing.
I also took a Teaching Elementary Science class, and got very interested in food chemistry. One thing that I clearly remember is the effect of salt on different things. We were shown how some vegetables cut and stored in plain water would remain crisp, but those stored in salt water became limp and rubbery.
For this hypothesis, you would probably write something about the chemical changes that take place in vegetables with the presence of salt. I can't be sure, but I'm trying to recall if placing those limp veggies (potatoes, carrots) in clean water didn't revive them. That could cover an entirely aspect of salt in vegetables...even searching for an application as to how salt affects food, digestion, etc.