Unless you are feeding a balanced high quality lab block as the only source of food, most of us like to offer our rats a little variety and try out new foods every now and then.
There are a few foods that shouldn't be fed or that should be fed with caution (you can find a list here: http://www.ratchatter.com/rat-food/forbidden-foods/), but otherwise, most of the things that we eat are safe for rats - although not all will be healthy choices.
Rats do not have the ability to vomit, so they can be very cautious when trying new foods. Just because they don't try something right away, doesn't mean they won't eat it. In fact, because rats do not vomit, they will frequently try a small amount of the new food and then wait a while to see if they begin to feel sick. They will then eat more later if there were no ill effects, or avoid the food if it did not agree with them.
For breakfast, we offer our rats a bowl of veggies, with a small portion of fruit as well. At first, our rats wouldn't touch grapes or blueberries. However, we found that cutting blueberries in half and grapes into quarters made a big difference. Once the juicy inner fruit was exposed, they were much more tempted to try them. And now that they know they like them, we can serve them whole again.
Also, remember that rats won't eat when they are full - so just because they reject a new food now, doesn't mean they won't come back to it when they are hungry. Leave the food with them and eventually, they may try it.