I have each grade once a week for after school, so we usually do a review game of whatever they've been doing in their morning classes, and it usually takes up the entire period. I start the classes by going over the key phrases and vocabulary (I have a review slideshow I do with them), then we play the game. I'd suggest you prepare backup material in case they do get bored, but I've also had classes stay after the bell to finish up a game, so really, whether or not they'll get bored is entirely dependent on their attitude. Bomb game is good because it's hard for a team to get and maintain a lead for very long. The same is true of any game where the questions are progressively worth more points (assuming the kids know this ahead of time).