Concrete nouns refer to material objects which we can see or touch.
Abstract nouns refer to things which are not material objects, such as ideas, feelings and situations.
The infinitive without to often emphasises the whole action or event which someone hears or sees.
According to the Cambridge Dictionary, “action” and “event” can be seen, which means, it seems, it classifies them as concrete nouns. But according to our common sense, they are abstract nouns and they can’t be seen. What’s wrong with the dictionary?