The below brainstorm and examples are for the general genre of gaming.  These could be games played on a computer, TV/Console, or mobile device.

Potential verbs

• achieve
• win
• tie
• lose
• play (already in Atom Activity Streams Base Schema) 

Potential objects

• achievement/Badge/Level/Reward
• game
• score
• player (already in Atom Activity Streams Base Schema as Person) 

Use Case: Achievement/Badge/Level/Reward

• <activity:verb> = achieve
• <activity:object-type> = achievement
• <activity:target> = game info

Example: Anna Apple achieved Underboss in Mafia Wars 

Use Case: High Score

• <activity:verb> = achieve
• <activity:object-type> = high-score
• <activity:context> = game

Example: Bob Bacon set a new personal record of 3:22 in Minesweeper 

Use Case: Two player competitive game result

• <activity:verb> = win (or tie or lose)
• <activity:object-type> = person
• <activity:context> = game

Example: Anna Apple beat Bob Bacon in Uno 

Use Case: Multi-player game result

• <activity:actor> = Potentially multiple person objects
• <activity:verb> = win (or tie or lose)
• <activity:object-type> = person (potentially multiple)
• <activity:context> = game

Example: Anna Apple and Bob Bacon beat Chris Cobb and Deepak Davis in Halo 

Potential Object Representations: Achievement/Badge/Level/Reward

Below are potential child elements of an <activity:object> representing an achievement/badge/level/reward

• <id> = Unique ID for that a/b/l/r
• <title> = Name of a/b/l/r
• <link rel="alternate" type="text/html" href="<URL_to_page_for_a/b/l/r>" ... />
• <link rel="preview" type="image/jpg" href="<URL_to_thumb-sized_a/b/l/r_image>" ... />
• (optional) <link rel="enclosure" type="image/jpg" href="<URL_to_full-sized_a/b/l/r_image>" .../>
• (optional) <summary> = Short description of what the user did to earn the a/b/l/r
• (optional) <content> = Description of what the user did to earn the a/b/l/r

Examples

Personal record:

Achievement/Badge:

Gaming: two-player competitive result:

Potential Object Representations: Game

Below are potential child elements of an <activity:object> representing a game

• <id> = Unique ID for that game
• <title> = Name of Game
• <link rel="alternate" type="text/html" href="<URL_to_page_for_game>" ... />
• (optional) <link rel="preview" type="image/jpg" href="<URL_to_thumb-sized_game_image>" ... />
• (optional) <link rel="enclosure" type="image/jpg" href="<URL_to_full-sized_game_image>" .../>
• (optional) <summary> = Brief description of the game
• (optional) <content> = Extended description of the game 

Notes on context

There are three potentially interesting aspects of gaming context:

• Game - Ex: Mafia Wars, Farmville, Halo, etc.
• Platform - Ex: MySpace, Facebook, Xbox 360, etc.
• Mode - Ex: Beginner, Legendary, etc.

The overwhelming most important of these three is the Game and thus it seems appropriate if we currently have one primary context element (<activity:target>) to map this to the Game.

Notes on players and teams

Especially as online and social gaming becomes more prevalent, variations in the number of players and teams/clans are becoming more prevalent, ex: 2 vs. 2, 8 vs. 8, Clan vs. Clan, multiplayer Cuththroat, etc.  Two-player competitive seemed like a good place to start.

Misc.

"The [online gaming] category attracted 87 million U.S. visitors in May 2009, up 22 percent versus year ago." - comScore