A Bonus Stage (also commonly referred to as a mini-game and bonus game) is a special mini-game found in various single-player modes in the games of the Super Smash Bros. series. They are used to interrupt the battle set-up, serving as a sort of "break" round. Completing these games under certain conditions can lead to the unlocking of various things, including trophies, stages, and even characters.
In these games, you are to complete a certain task (determined by whichever game you're playing) within a certain time limit and with only one life to spare. Bonus games are often designed so that they feature custom arenas, designed specifically to test your skill with a certain character.
Bonus games can be played in one of two ways: as mentioned before, bonus games are incorporated into various single-player modes. Additionally, each bonus game (excluding Race to the Finish) can be played directly from each game's menu.
A notable feature of most bonus games is that they are entirely optional (as far as being able to progress through the game); failing one of these games will not result in a loss.
In Super Smash Bros.
Three bonus games are available in the 1P Game of Super Smash Bros. The first, which can be played only after winning the first three battles, is Break the Targets. In this mini-game, you attempt to break all of ten targets scattered throughout a special arena. After completing this mini-game (either through winning or losing), you are allowed to progress further through the main game by fighting in another three battles. When the next set of battles has been completed, you are allowed to play Board the Platforms, a mini-game in which your objective is to land on ten platforms located in various locations of yet another special stage.
The final mini-game, Race to the Finish, can be played after finishing the third set of battles. This mini-game, unlike the previous two, features one universal stage for all characters and is not designed to play on any particular character's strengths and/or weaknesses. Also unlike the previous games, Race to the Finish must actually be successfully completed in order to continue on with the main game.
In Super Smash Bros. Melee
As in the original Super Smash Bros., Super Smash Bros. Melee also features three bonus games. While Target Test (previously "Break the Targets") and Race to the Finish return, Board the Platforms mysteriously doesn't, much to the dismay of its fans. In its place is a new mini-game: Snag the Trophies, which ties into the trophy-collecting aspect of Melee. The concept of this mini-game is self-explanatory: your goal is to collect three trophies that fall from the sky by knocking them onto a large column in the middle of the arena. Just as with Race to the Finish, Snag the Trophies does not feature specialized stages for each individual character, instead using one generic arena.
In Super Smash Bros. Brawl
So far, only one bonus game has been confirmed for Super Smash Bros. Brawl: Target Test, reworked and renamed (for a second time) as Target Smash! Several changes, ranging from minor aesthetic alterations to complete functional changes, have been made. For one, no longer does the mini-game feature individual stages for each character; instead, there are five generic stages (of varying difficulty) that all characters can play on. Secondly, some of the maps have been confirmed to feature items - a first for bonus games. Finally, players can now record footage of their attempts at completing the mini-game.