Almost all food-related nouns (including drinks) are uncountable. The uncountability of those nouns comes from the fact that they are material and some grammar books call it a material noun. There are millions of materials in the world, but strangely enough, English doesn't feel the need to count them because (more often than not) it is impossible and sometimes meaningless to count them. In addition, there are words to help you when you count them, e.g., piece (for many uncountable nouns), loaf or slice (bread), cup (coffee and tea), glass (beer and water), etc.
For example, you can have two eggs (countable) and bacon (uncountable) for breakfast. You can count the eggs, but you have to use piece or slice to count bacon. The reason is bacon on your breakfast plate was sliced off one block of meat (which is called bacon). How many pieces can you slice out of , let's say, 500 grams of meat? You could slice 100 pieces, but I could 200 pieces because I need to eat it for a longer period of time. Then, is there any difference between 100 pieces of bacon and 200 pieces of bacon in terms of their weight? Not a single gram of difference. That's the basic characteristic of a mass noun. You can cut/divide it into pieces, but the pieces you see have the same quality (appearance, smell, taste, etc.) as the original one.
You have a cup of coffee. You want to share it with your friend. You pour a half of your coffee to his/her cup. Is there any difference in coffee between the coffee you had originally and two cups of coffee you two are drinking now? There is no difference in terms of smell and taste. Only difference is quantity which is uncountable.
The same rule applies to breakfast. When food items for breakfast are combined, you can divide it by half and share it with your friend. Then, are they called half-breakfast? No, the two plates still have breakfast with the same smell and taste.
When you think about a mass noun, it is very important to have this concept. Of course, someone can argue "You can count the number of fishes on your plate, how come fish is a mass noun?". Well, my answer would be "Fish is an extraordinary case". There is countless number of fish in the sea, but your fish for breakfast is countable. However, if you cut one fish in the middle, what do you see? It is still fish. (Fish has countable and uncountable usage)
I don't agree with calling breakfast as an activity. There is no reason to classify it as one. It is just one of your concepts that's conjured up when you think about having breakfast. In this case, it is closer to an abstract noun. But, the important question is "Why would you call something you can eat, touch, smell and even throw away an abstract noun?" There is no reason.
When you say, "Are you free for a movie, Star Wars, tonight?", the noun movie is a common and countable noun. However, you are talking about an activity of watching the movie. Then, does a noun movie become an abstract noun? Not necessarily so.
Note: This answer can never cover the difference 100% among various (mainly five) types of English nouns. However, there seems to be a concrete rule governing countability and uncountability of nouns.