Carbonated beverages, diet or otherwise, are a bad idea because they can leave you feeling bloated, gaseous, and jittery with a sugar or caffeine rush. Iced tea can also keep you up all night with pre-race jitters, and the caffeine has a dehydrating effect as well.
With pure water, you stay hydrated without additional calories, and there will be no unexpected side effects that a fruit juice, dairy product, or soda could provide.
Snacktime
Snack foods are a mixed bag. Pretzels, the longtime "healthy" snack favored by junk-food-junkie athletes, are a fine carbohydrate provided that they are unsalted (salt causes your body to retain water, leaving you bloated and heavy).
Dried fruits, although they sound natural and healthy, are bad because they pack plenty of calories and, once consumed, they absorb water and expand to their full size (so just a few dried apricots can bloat you). Granolas and nut mixes usually contain lots of fat and can result in inopportune trips to the john.
The freebie race bars and snacks that one finds in goody bags and pre-race registration booths may be fine in small doses, but don't assume a product is healthy just because it is in your goody bag, and don't overeat just because the products are there, free and in unlimited supply. Read the labels!
Light is Right
The morning of your event, it is best to eat light. Avoid a hearty breakfast and go for half a bagel, half a banana and a glass of water. That always works for me in long-distance ocean races and for the few marathons I have run. Of course, during a race it is important to stay hydrated and carbo-loaded, but I am a light eater when I compete, and generally just drink water and intake a PowerGel pack every three miles or so in a swim, and perhaps only twice during a marathon.
In sum, it's best not to change your diet too drastically in the days before you race. Increase your carbohydrate load slightly while maintaining a healthy intake of protein, and stay away from dairy products and highly acidic fruits.
Twenty-four hours before your event, I suggest drinking water for fluid intake and having a light meal at dinnertime the night before consisting of grain-based carbohydrates, soft vegetables, and light protein (chicken, fish). The morning of your race: water, half a bagel, and half a banana should suffice in providing sustenance and optimum performance without weighing you down.