Burial site or playground, jobs can always be found for one willing person who sets up an appointment in advance. What's ideal for the garden supervisor, though, is a small group of friends who come with esprit de corps and a picnic lunch.
Remember that this is the gardener's precious workplace, full of tender living things he or she has nurtured. Try not to step on stuff. Do not complain about picking up trash. In an urban park, a gardener often spends the first hour of his or her workday plucking litter from the flowerbeds.
Remember that plants and trees are living things that must have a few needs met if they're to thrive. Bulbs have to be planted deep enough (the rule of thumb is three times their height), with the roots down and the tips up. Ask the gardener if you're not sure, when planting shrubs or small plants, what's too deep and what's too shallow. When you're spreading mulch, take care not to pile it around the crown of a plant or over the bark of a tree.
Should you come with a Cub Scout troop or your child's class, be sure there is one adult present for every three children. (Perhaps you note here some bitter experience.) Those adults must set an example (no texting or talking on a cell phone) by listening to the gardener's instructions. They must also keep the kids from throwing things at each other (tulip bulbs, worms) or using a hose in an inappropriate manner.
When you've gained some park experience, you might want to consider some more sophisticated tasks. Think about working in a non-Western garden. Many big American cities have Japanese or Chinese gardens. There's a famous one in Portland, Ore., and a hidden treasure on Staten Island, the Chinese Scholar's Garden. In gardens like these, you take in centuries of tradition. But what the 15th-century Chinese garden designer Ji Ching said still applies to gardens planted this season: "The garden is created by the human hand, but should appear as if created by heaven."