Rocks is a playground game that I learned in high school. It also goes by rubber, or sand, or whatever happens to be underneath the playground. The game Rocks is most similar to is tag.
In a game of Rocks, one player, generally the last player to reach the playground is designated it. The it player, who I'll describe as the chaser to avoid using random pronouns(but is simply called "It" during gameplay), moves away from the playground. The chaser closes his or her eyes, counts to a number, generally ten, and then begins to head for the playground to attempt to tag a player. The chaser must keep their eyes closed at all times. At any time the chaser can call "rocks!" and if any of the other players are on the rocks, they become it. Otherwise, it plays basically like tag, with it having their eyes closed.
One of the interesting parts of rocks is jumps. If you jump the person who is it, you get a point. Points are useless, except whoever is your fifth jump has to buy you an ice cream cone. This applies to all multiples of five. This rule is primarily an excuse to get rewarded for the crazy acrobatics that are part of a common game of rocks.
Rocks is a game that's playable at all times you have a few people together, and can be played on an improvised playing field. One word of advice is to be aware of people who are not in the game of rocks. Small children can ruin a game, as well as angry parents of those small children. Always have a backup plan. Generally, it's wisest to call your backup plan "Shenanabobo!" so you can scream it, jump into cars, and drive off somewhere, leaving the threat to the game behind.
Some advice from an experienced rocks player:
Playing rocks under the influence is generally a bad idea.
You will get kicked in the face. Laugh and write a song about it.
Bring food. This is a good way to not be it at the start of a session.
The game is best with a few close friends and a few strangers. You'll probably end up friends with all of them anyway.
Most of all, it seems like a silly game, but give it a chance. It's a rewarding way to waste some time.