A very long post. Sorry I could not make it shorter.
I've searched on the Web for relevant information, and I learned two things:
1. As you know, when all the folks who put cans of regular ("Original" I think) Coke in water, the cans sank. This contradicts your experience. First hand data takes precedence over information found on the Web, so it is within the bounds of possibilities that the other sites that "explain" this experiment have not actually done the experiment, but just copied some other Web information. I have seen this happen. However, several sites have photographs of their submerged Coke can, so unless they have all faked their results to get the "right" answer (for example by adding a weight that can't be seen in the photograph -- a rather extreme hypothesis!!) their experience contradicts yours. We are left with a mystery. Could it possibly be that your can of regular Coke is not the kind of Coke with all the extra sugar? Check the ingredients table on the side of the can -- from Web discussions it seems that regular coke has something like 49 grams of sugar per can, so if your can shows only a small amount of sugar it may be a formulation that differs from "Classic" Coke. I am unfamiliar with the available kinds of Coke and their appearance since I do not drink sodas, so it's hard for me to tell how easy it would be to get the cans mixed up. If your can does indicate a lot of sugar, >10 grams say, then it must be a "regular" Coke and then we are faced with a flat-out contradiction (or the very extreme hypothesis put forth above! (or something I haven't thought of)).
I shall assume that you have already repeated your experiment several times with the same result.
As a scientist, your next step then should be to get someone you know to perform the test of whether regular Coke sinks or (as for you) floats. (They should buy their own can of regular Coke.) If they get a different result from yours, then you can zero in on exactly what is different in what they did in this test versus what you did.
If you both get the same result, contradicting the information on several Web sites, then you should attempt to contact the authors of the Web site content to see if they can throw any light on your experience.
If you find some major problem with the way this experiment is presented on the Web, your final step would be to write a paper and submit it to a science education journal.
I know that this train of action is extreme, and it is unlikely that you have the time and/or interest to pursue it. But that is what a scientist does "for real". Meanwhile you should present your data AS IS to the science fair, including if possible a live demonstration of a submerged can of regular Coke. Should make for a really interesting presentation!
2. It has been a very long time since I took a class in chemisty, but from reading on the Web, especially this URLhttp://www.newton.dep.anl.gov/askasci/c ... m03838.htm
I have concluded that the explanations of why a solution of sugar in water is more dense than plain water given (at least tacitly) in these Web sites are all bogus. You might think this unlikely, but it would be far from the first case where a simplified "psuedo-explanation" given in text books and other educational contexts, is actually quite wrong. You can find a few examples athttp://www.ems.psu.edu/~fraser/BadScience.html
In truth, one cannot determine the density of a water/sugar solution except by actually measuring it (see the first URL given for the details). An example of a completely non-intuitive result is that adding 18 mL of pure ethanol to a liter of pure water a room temperature only increases the volume of the mixture by 14 mL! Four mL have "gone missing". Turns out that they are hidden in a sense within the molecular organization of the solution; the solvent allows the water molecules to pack together a little bit better than for the case of pure water. So the combination is less than the sum of its parts. Seehttp://en.wikipedia.org/wiki/Partial_molar_property
for more details.
Whew! This long post is over. Just remember, the truth is everything
Proud to be a Scientist