Experiments with water work great when being asked to not work with animals. How about this: examine the time it takes to heat water to its boiling point? The hypothesis is relatively easy, and the multiple trials should deliver fairly consistent results. Also, the data graphs nicely and presents well.
I really like the idea of some of the practical life experiments above. An interesting experiment that my class did once was on water filters for drinking water. We compared the 'control' group (tap water) to different various methods of filtering water to see which came out the purest. We also compared some of the different bottled water brands.
What about testing a hypothesis like--students who exercise for 15 minutes score higher on a quiz, than those who do not. You could get a heterogenous group of students--let's say 50, and give 25 of them(experimental group) some exercises to do for 15 minutes and the other 25(control group) no exercise and then administer the test. See what kind of scores you obtain(your data) and plot these numbers as a line or bar graph. Draw conclusions about your hypothesis and report your findings!
You may look at flipping some of the concepts suggested above by reversing the process - compare melting rates of different types of liquids; melting rates of the same type of liquids when placed in different temperature environments; etc.
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.
You could explore the diffrence between dissolving (which needs a solute and a solvent) and melting (which requires only the substance and heat). You could catalogue the melting and dissolving points of common substances, and see if ther are any which do both.
You could examine the temperatures by which different liquids freeze or boil. Use sodas (regular, diet, clear, and colored) or different juices (apple, grape, even tomato). My son did this experiment one time and had a blast.
You can also experiment with container shapes, makes (plastic/metal), and lids.
What about getting cheap paper towels and more expensive paper towels. Cut squares of the same area. Then weigh those squares. Hypothesize that whichever paper towel has the greater weight per area will be more absorbent. Test by dropping water from an eyedropper (or something else that will give you a measured output) onto the towels while they are suspended in the air (use a rubber band to secure them around the top of a cup or something). See how many drops each one can take before it allows water to leak out.
How about using plants? You could get plant seeds and record how fast they germinate at different temperatures; for instance at room temperature, in the refrigerator, and in a warm place (on a heating pad or under a desk lamp, for instance). If you do this experiment, be sure you put all the seeds inside something so that they are all in the dark, and give them all the same amount of water. A good setup would be to find three jars that are all the same, put a damp paper towel in each, add the same number of seeds to each jar, wrap each jar in aluminum foil, and put them in the three locations.
This experiment could be done cheaply by buying a container of whole mustard seeds at the grocery store and using them - they grow really fast, and there are a lot of them in a package. If you can't find mustard seeds, a bag of whole dried peas will work instead.