Super Smash Brothers Brawl starts with a locked set of options and a small amount of characters available for play. Normally, a player would go through the single-player adventure mode called The Subspace Emissary to collect new characteres to use in the normal game, but it's unlikely that there will be a group that's willing to let only one or two people play the game.
The game wisely avoids forcing people to play one mode to get everything, and so it will also offer unlocking of characters, stages, and other options on a schedule of matches played, knockouts scored, events cleared, and stages played on.
The fastest way to arrive a fully unlocked system is as follows:
- Select the Special Brawl option from the main menu.
- Choose the "Stamina Match" and "Flower Head" options.
- On the character selection screen, set the CPU (or another player) to have 1 HP.
- Play 10 matches as Donkey Kong on the Halberd stage. [You'll unlock Ness and Marth when you defeat them at 5 and 10 matches played, and the stage Frozen Hillside at match 10.]
- Play 10 more matches as Donkey Kong on the Pokemon Stadium 2 stage. [You'll unlock Pokemon Stadium and 75M.]
- Play 10 stages as Kirby on the Battlefield stage. [You'll unlock Luigi at battle 22 and the Jungle Japes stage at battle 30.]
- Play 10 matches as Kirby. [You'll get Green Greens at the end of this set.]
- Play 10 matches as Luigi. [You'll get Luigi's Mansion at the third battle, and Falco appears at the end of battle 50.]
- Play 20 matches as whomever you like. [Captain Falcon appears at the end of match 70.]
- Play 10 matches as Captain Falcon. [You should get Big Blue and Port Town.]
- Play 20 matches as whomever you like. [Lucario will appear at the end of match 100.]
- Play 350 more matches as whomever you like. [Snake is match 130, R.O.B. is 160, Ganondorf is 200, Mr. Game and Watch is at 250, and he carries Flat Zone 2 with him, Sonic is at 300, and he has Green Hill Zone accompanying, Jigglypuff is at 350, Toon Link at 400, with the Pirate Ship accompanying, and Wolf is at 450.]
- Go to the Single Player mode, and choose the Event Match option.
- Complete Events 1-28. [#19 is Mario Bros., Spear Pillar is #25, and Hanenbow is #28.]
- Only one thing left to go! We need to accumulate 200 KOs in the normal Brawl mode to unlock More Options. To be sure we get all the stages, play first as Captain Falcon on Halberd for 10 matches, then Donkey Kong for 20, first on Pokemon Stadium 2, and then on Battlefield.
Congratulations. You should have all the stages, characters, and options unlocked. If the guide is correct, you should see that you've unlocked the "Random Stage Switch" option in the "More Options" menu. You will probably want to turn off including the Custom Stages in random stage selection. The kids might also have opinions about which stages they do or don't want, as well.