A lot of otherwise empirically-inclined people think they can dismiss Creationism by an entirely nonempirical and a priori argument. They say that it isn't falsifiable and therefore that it cannot be considered by science. But Creationism is falsifiable. In fact, it has been falsified. Here are five general observations which could potentially falsify Creationism: (1) the nature of the fossil record, (2) the flexibility of reproduction, (3) the beneficial quality of mutations, (4) the possibility of making life from non-life, and (5) the age of the Earth. I'll discuss each in turn.
(1) Creationism postulates that God created individual creatures whole and that he created them to reproduce after their own kind. This postulate could be falsified by the fossil record. If the fossil record were to show relatively few long-lived distinct species and relatively many chains of species where one is clearly developing into the later species, then this would tend to falsify Creationism. A creationist would expect to find primarily distinct species, repeated at many sites, and very few apparent chains of ancestry where one species seems to be developing into a later species. The fossil record in fact shows 0 such chains of ancestry and many distinct species that lived for millions of years.
(2) Creationists believe that animals reproduce after their kind (a kind may or may not be a species) and that they cannot evolve into a different kind. This belief could be falsified by taking a some rapidly-reproducing species and using selective breeding and (possibly) the use of artificial mutation to artificially "evolve" a new, viable kind. I'm not talking about breeding within a species like breeding wolves into little yippy dogs; what would falsify Creationism is a demonstration of a distinctly new and healthy species with distinctly new and functional structures. To really be convincing, the new species must be more complex than the original species.
(3) If you can't breed a new kind within a few years, you could at least make it plausible that it is possible for a new kind to evolve by showing mutations that are beneficial to the organism. Nearly all significant mutations are harmful to the organism (at least for higher-level organisms), but if you could show a process that produces some reasonable chance of a significant and favorable mutation, that would tend to falsify Creationism.
(4) Creationists believe that only God can create life from non-life. You could falsify Creationism by demonstrating the creation of life in the laboratory --not the creation of amino acids, but the creation of living cells.
(5) If it were possible to prove that the Earth is older than 6,000 years, that would tend to falsify Creationism. But wait! Radiological dating has proven that the Earth is older than 6,000 years. By doing so, science has falsified Creationism. Creationists had to adapt and change their theories in response to this technology. There are still some young-Earth creationists, but they are not nearly as dominant as they once were, and those that still exist do have some rational, maybe even plausible, arguments for not accepting the results of radiological dating. They don't just ignore the scientific evidence, they address it on empirical grounds just like a scientist would do.
Wait, if Creation is falsified, doesn't that mean it is unscientific to continue to be a Creationist? No, scientists very seldom give up a broad theory on the basis of a few falsifications. Take Evolutionism for example (I'm using "Evolutionism" in contrast to "evolution". Here, evolution is the theory that species can evolve by natural selection; Evolutionism is the theory that all life originated and evolved through purely physical processes). Of the five observations I listed above, one of them has falsified Creationism and the other four have falsified Evolutionism. In each case, Evolutionists predicted the opposite of what Creationists predicted, and so far, a century and half later, the Creationists are proven right, four out of five.
Does this mean that Evolutionists are compelled by the scientific method to give up their theory in the face of falsifying evidence? No, science doesn't work that way. When observations don't come out like you expected, you don't just throw out your theories, instead, you make the minimal changes to your theories that are necessary to make them consistent with the observations. For a long time, physicists accepted Newtonian mechanics even though it failed to adequately explain the orbits of a couple of planets. Eventually they discovered that one of the orbits could be explained by the existence of another planet. The other one they never did explain. Then Einstein came along and threw out Newtonian mechanics. So, eventually the theory was abandoned, but it was kept around for two centuries after it was falsified.
But let's be honest. Creationists have a strong non-empirical reason to believe their theory and this is going to trump just about any observations. It's hard to imagine an observation (other than time travel) that would make committed Creationists give up their belief in Creationism. But as long as we are being honest: honestly, what observation would turn a committed Evolutionist away from Evolutionism? It's equally hard to imagine. The truth is that Evolutionists don't believe in Evolution because of the overwhelming evidence; they believe in Evolution because they can imagine no alternative that is acceptable to them. It is no different with Creationists.
But there is a difference! Creationists can always appeal to miracles to get out of any observations! That makes them unscientific! Well, of course, if a Creationist uses the "God is just playin' with you" defense, then that Creationist has taken himself out of the realm of scientific discussion and can be reasonably ignored. There are some Creationists that have done this. For example, some Creationists argue (I use the word loosely) that God put fossils in the ground just to make the Earth appear old. I am by no means defending this form of Creationism. That truly is unfalsifiable.
But most Creationists do not appeal to such sledgehammer tactics and it is not reasonable to ignore the reasonable Creationists just because there are unreasonable Creationists. Most Creationists are willing to make substantial modifications to their beliefs in response to evidence. A century ago, most Creationists believed the Earth was only about six thousand years old. Today, I think the majority have accepted the findings of radiological dating and have made major changes in their understanding of Genesis. As long as Creationists are willing to engage in scientific debate, only a religiously committed Evolutionists would refuse to debate them.
UPDATE: From Dean I just found this article about mice that can regenerate. The mice were discovered by accident, and if their genetic advantages turn out to be a mutation, that's another one for the Evolutionist side.