It doesn't have to be raining out to turn your garage into a playroom for your kids. In fact, there are some games that practically can't be played nearly as well if they are played anywhere else. Of course, having fun in a garage also means you'll have to clean it up first.
Dressing and Daring
Have kids play a game of pretend. Take a bunch of old clothes--stuff from others in the family, Halloween costumes and the like--and pile it all together on the garage floor. The let the kids turn themselves into whomever or whatever they like. You can even challenge them to come up with a skit to perform for you and others that incorporates their new "identities." As long as you warn them off any potentially dangerous items, you can also encourage them to use things found around the garage as props for their performance.
If you have a few minutes to plan ahead, you can create a simple but fun obstacle course running through the garage. Draw a chalk line the kids have to follow by placing one foot in front of the other. Have them leap through the rungs of a ladder placed on the ground and over paint buckets. Have them throw a rubber ball into a laundry basket or the center of an old tire. Put it all together and see who can get through the course the fastest.
Buy or make your own pinata (see Resources) and string it from the garage beams. Depending on the age of the children, you can do it traditionally and blindfold each player, or leave them unblindfolded if they're very young. If you have a mix, blindfold some to make the game more even. Spin each player around at least three times before they take a swing.
Hand the youngest player the bat or stick and let him take the first crack at the pinata. Then let the next youngest try, and so forth, until someone pops open the pinatas and the treats come pouring out.
If you'd rather not have kids swinging a stick, you can buy or make a pull-string pinata. With these, there is a trap door in the pinatas that only opens when the right ribbon--there are usually at least a dozen other ribbons on it--is pulled. So, like before, have the children take turns, only this time all they have to do is pick a ribbon and pull it.
Let the kids play a game of indoor H-O-R-S-E by placing an empty paint bucket against one wall. Hand one player a tennis ball or rubber ball and let him take his shot from anywhere and in any way he wishes. For instance, he might try to bounce it once before it goes into the bucket, or set up a ramp and roll the ball so that it climbs the ramp and falls into the bucket. Anything goes--the more creative the better. The basic rules of the game remain. If the first player misses his shot, the next player can shoot from anywhere. If the first player makes his shot, the next player must make it, too. If he misses, he get one letter (in order) from the word "horse." Once a player earns all five letters, he's out.
For a little quieter game, teach the kids dice baseball. Give them a pair of dice and let players take turns rolling the dice against the garage wall. The number each player rolls determines how well he "batted." For instance, a 12 might be a double, a 2 might be a homerun, and so on. At least half of the rolls should be outs, so that it's not too easy to score runs. You can decide ahead of time what each number represents and then make a scorecard for the kids. You can divide them into teams and let them play against each other, just like in real baseball. One team continues to "bat" (roll the dice) until they make three outs, and their score is written on the sheet. Then the next team bats. Play a full nine-inning game or play shorter, depending on the ages of the kids and any time constraints.