o understand what Activity Streams is, think of it as an abstract syntax to represent basically anything that can be an action on social media. The Activity Streams Vocabulary specification defines, amongst other things, three types of objects: Actors: Application, Group, Organization, Person, Service. Activity types: Accept, Add, Announce, Arrive, Block, Create, Delete, Dislike, Flag, Follow, Ignore, Invite, Join, Leave, Like, Listen, Move, Offer, Question, Read, Reject, Remove, TentativeAccept, TentativeReject, Travel, Undo, Update, View. Objects: Article, Audio, Document, Event, Image, Note, Page, Place, Profile, Relationship, Tombstone, Video. To build a valid Activity Streams activity, you pick one of each category and add some metadata to it. You describe that something did something to or with something, and you explain those things in more detail.
A valid activity in Activity Streams is using one of each Actors, Types and Objects. Me Arrives at Place, Me Travels to Place, Me Announce Event etc. It's all JSON