'stuff' is a very informal word that takes the place of listing specific things. In your case, it refers to various activities, but it can also refer to various items, or just as a general way to avoid being specific. For example: "What were you two talking about?" "Stuff." This makes it clear that you do not want to answer the question. However, 'stuff' has no place in formal writing or professional conversation. It's seen as lazy and weak in any sort of academic writing, and a little rude in professional conversation.

In this example, it means "something or other". A very general and casual way of saying "things".