Tomatoes are questionable for ducks. Having only five little ducks I'm not going to feed what might hurt them. I've always fed tomatoes to the chickens and turkeys. But, with so many other foods, the chance isn't worth taking.
Also kale, collards, cabbage, chard, lettuce and all kinds of squash are great treats. Root vegetables such as sweet potatoes, beets, turnips, carrots, radishes and parsnips are also super nutritious treats but ducks have a far easier time eating them if they are cooked or grated. No tomatoes or...
My call ducks favorite treat is dry meal worms sprinkled in their drinking water and little pond (kiddie pool). They also like moistened dry dog food and hard boiled egg. Peas too.