When it's raining outside, you have take physical education, or PE, class inside. These are the most reliably fun games that kids can play inside of a gym. Remember that if you're playing with rubber balls, there is a chance that a kid can hurt himself or another kid with it. Be prepared for any minor injuries when playing these fun indoor games.
This is a game of tag designed specifically for play on a gym's basketball court. It gets its name from the old Pac-Man arcade game. The players all have to walk on the lines of the court, be it the three-point line, halfcourt line, out-of-bounds line or free-throw lines. If the player walks off of a line, he'll be out of the game entirely. One player is "Pac-Man" and travels on the lines, tagging those who can't get away from him in time. When they are tagged, they must immediately sit down and remain in that spot. They now become "ghosts" and can tag any walking player who gets too close. The last walking player who hasn't been tagged by "Pac-Man" or a ghost is the winner, and he becomes "Pac-Man" in the next round. This game is for ages 6 to 9.
This is a grade-school favorite. Place five to 10 rubber balls along the halfcourt line of the gym. Line up half of the kids on one side of the court, and the other half on the other side. These are the two teams. When you say "go," the kids on both sides run as fast as they can to the halfcourt line and grab the balls. Each team must stay on their side of the halfcourt, and throw the balls at the kids on the opposing side. If a ball is caught, the thrower is out. If the ball bounces off of the kid and he doesn't catch it, then he is out and the thrower remains. This is a team game, so a side wins once all of the kids on the opposing side are out. When a kid gets out, simply have him stand beyond the sideline so he can keep cheering for his team. Line up the children in the order of who gets out first, because he can get back into the game if someone from his team catches a ball. Make it a rule that players can't throw balls at another's head. This game is for ages 9 and up.
This game can also be played outdoors at a baseball diamond, but is fun for younger kids when played in a gym. Use rubber bases. Place home plate at one of the basketball court's corners. Place the other three bases at the distance you think will be appropriate to the age of the kids playing. Play this game exactly like baseball, but use a rubber ball instead. Don't use bats, make the "batters" kick the ball as it comes over home plate. This game is for ages 6 to 9 (for indoors), ages 9 and up should play kickball outdoors.