My students always complain that we don't play enough games in class. I know they love games, but most of the games I've seen are quite ineffective. "Showdown" is one of my favorite review activities for my older (more mature, more motivated) students in Algebra 2, but it doesn't work so well for my freshmen in numeracy.
To many of my students, tic tac toe is a riveting activity to be played surreptitiously during a dull lesson, or after a test. I thought I would capitalize on that, and so I present to you:

The students are broken into pairs that collaborate against other pairs. Each group of 4 is given a game board with several tic tac toe grids on it. One pair picks X and the other gets O. Turns alternate with each problem. When the problem is shown, both teams should work on it. If it is X's turn, if they are right, they get their square. If they are wrong, and the O's are right, the O's get to steal a square. (Students took a while to get this - at first they all thought it was unfair). When a game is won, the winning team gets a point. At the end of the activity, whichever teams won more games get a prize.
The benefits:- All students are engaged on every problem. Even if it's not their turn, they can steal if the other team is wrong.
- Students have a partner to collaborate with, so weaker students are not put on the spot and can learn during the activity.
- Pairs monitor each other for cheating - they can only get the square if they've shown their work.
- Tic tac toe is the funnest game on the planet. Apparently.
- Generic mechanical pencils in fun colors come 30 to a pack for $5. Great prizes! Mini candy bars work too.
Enjoy! Let me know if you play it and it works (or doesn't work!) for you.