Here's a variation of UNO that I invented just for Maxicon and playtested there to great success!
It is called "DOS"
Players: 6 or more (doesnt work with less)
Split into two teams by counting off aroud the circle. First player says "Uno". Second says "Dos. Then "Uno", then "Dos" until you have two teams whose members alternate aroud the circle. Team Uno, and Team Dos. Team Uno players must sit, while Team Dos players must kneel. Trust me -- it will help! Team Uno will play a normal game of UNO. Team Dos will play the opposite of Uno. That is to say, when playing a card, they must NOT match the color and NOT match the number (or word) on the previously played card (the card on the discard stack).
Play this slowly at first, alternating players. After turning over the first card, the first player (playing UNO) matches the top card either by color or number. The second player (DOS) must NOT match the color and NOT match the number. If a player cannot play, they must draw until they can play as per normal rules.
A REVERSE reverses play. A SKIP skips the next player (who is on the other team). A WILD DRAW 4 forces the next player (who is on the other team) to draw 4.
When a player runs out of cards, he sits back from the playing field. Once the last member of a team goes out, that team wins! However, you can "bring" a member of the other team back in the game by playing a "DRAW 2" or "WILD DRAW 4" because it affects the next person in the circle.
That was DOS.1 Once you get the hang of it. Move to the next version, DOS.2
In DOS.2, each team plays on the same discard (like above) but each team is not playing separately from the other team. In other words, when you flip the top card to start the game, either of the next two players can start playing. Once a player on a team plays, the NEXT player on that team can then play. The play does not alternate between teams necessarily. Team Uno might play twice as fast as Dos. HOWEVER, they both are playing on the same discard stack. This means that the card you are trying to match might change several times as you are trying to play. In fact, if you dont have a card to play, you can draw until you do, but you might also just wait (or draw slowly) because the top card will change as fast as the other team plays.
In DOS.2, the SKIP and WILD cards affect the next player (who is on the other team) like above, but a REVERSE card only affects YOUR team! Do realize however, that the next player in a WILD DRAW 4 or a SKIP might NOT be the next player to play even though they ARE the next person sitting aroud the circle from you. You will need to yell loudly at them "Draw 2" or "I skipped you!". This makes it confusing for the next player who has to be on his toes. And who must remember to be "skipped" once it's his turn. More chaos is always better!
Finally, are you ready for DOS.3 ?
In DOS.3, each WILD card makes you change teams. Yes. When you play a WILD, yell "Switch" and if you are kneeling (Team Dos), sit down and become a Team Uno player. The next player must now switch the team HE is on. However it does get crazy when all of a sudden two players from the same team are sitting next to each other (both end up kneeling or both sitting). In this case, when you make them DRAW, it might bring your OWN teammember back into the game!
If you want, you can have a WILD DRAW 4 make both teams swap. All kneelers sit and all sitters kneel!
It's as simple as that. Yelling. Pretty colors. Numbers. Chaos.