Ephemeral Diversion

An ephemeral diversion is a typically benign segment of a videogame that offers an unexpected level of interactivity.

Examples

Katamari Damacy
Punch-Out!! (Wii - the player is able to use the cursor to 'correct' misspellings and remove the names of the fighters)
Super Smash Brothers Melee

Assassin's Creed (player can run around and jump in a lobby)
Bayonetta (player can practice combos)
Castlevania: Symphony of the Night
Katamari Damacy (some versions offer minimal interactivity on the loading screen; analog sticks move stuff around)
Mass Effect (of its two types of loading screens, one allows the player control of a camera within an elevator)
Okami
Onichanbara vorteX (player can move a 2D chibi avatar around and cut up a horde of 2D chibi zombies)
Phantasy Star Online
Super Smash Bros. Brawl (before online matches, player can beat up a sandbag)

Gunstar Heroes (the title screen logo can be rotated if down is held on controller one before it appears)
Iridion II (the music's instruments can be changed)
Metal Gear Solid 3: Substance (the color scheme and background image can be changed)
Meteos
Puyo Puyo Fever (DS - the player can attempt to bounce a Puyo in the air with the touch screen for as long as possible)
Punch-Out!! (Wii - the player is able to interact with Mac using the pointer)
Ridge Racer Revolution
Star Fox 64
Super Mario 64
Warioware: Smooth Moves (the player can tug Wario's mustache with the cursor and wave the remote to cycle images)
Warioware: Touched (the player can interact with numerous objects, such as a ball that leaves rainbow trails onscreen)

Super Smash Bros. Melee (all menus in the game are rendered in 3D and can be tilted with the C-stick, showing that they are panes hanging in air)