Scavenger Hunt
One of the easiest outdoor games to organize is a scavenger hunt. Simply produce copies of a list of items that participants need to collect, and then form teams, and send them out to attempt to find every item on the list before other teams do. A scavenger hunt can last anywhere from a half hour to an entire weekend.
Tailor the items on a scavenger hunt list to the environment. For example, in the forest, ask players to find different types of leaves and rocks. At the beach, a scavenger hunt list may include shells, plants and even live animals like sand crabs. To set up a scavenger hunt that doesn't threaten nature, give each team a camera and require photo evidence of each find instead of the actual item.
Croquet
Croquet may seem like an old-fashioned game for high society types, but it is easy to learn and requires only some simple equipment. A set of mallets, balls and hoops can easily fit in a car trunk, making croquet appropriate for playing at a public park or a neighbor's yard.
Most croquet sets come with a copy of the official rules, which are simple. The act of hammering a ball with a mallet will be familiar to most children, and older players can enjoy using strategy to form more complex game plans. Croquet is also slow paced and easy enough for elderly players, making it a good option for family reunions where multiple generations can all play together.
Bigger and Better
Bigger and Better is an outdoor game that works best outdoors in a city or village environment. Each team of players starts out with the same small object, which can be anything from a paper clip to a pencil to a shoelace. The goal of each team is to trade the object for something bigger and better with members of the public.
The game has a time limit, usually around two hours. Each team continues to make trades, and whichever comes back with the object judged the biggest and best at the end of the time limit wins. Bigger and Better is only appropriate for older children who are comfortable being alone in public and in safe neighborhoods during daylight hours.
Races
Racing is one of the simplest outdoor games, requiring no equipment and appropriate for all ages. However, you can create a more involved relay race for larger groups or participants of different ages. A relay race consisting of a foot race, a bicycle portion and a swimming leg takes the form of the classic triathlon. Consider also adding a three-legged race with participants tied to a partner or a sack race with "runners" hopping inside a large bag. Plot a course to take advantage of your environment, and add portions of the race that are most appropriate for the age and number of participants.