the 40th Anniversary of Earth Day a valuable gift you can share with others (especially children) is the simple ability to observe.
In fact, if I had only one bit of advice on encouraging an interest in the natural world, it would be this: foster keen observation and encourage curiosity.
If you're thinking "... but, how?" well, you're in luck: getting started is easier than you think!
First, get outside and into close contact with the natural world. This should involve dirt, maybe a little mud, plants, insects, rocks, untreated water, and a distinct lack of paved surfaces. Fortunately for most of us, this is as easy as heading into the back yard or to a nearby park or natural area.
Second, bring along tools that enhance the senses: magnifying lenses, binoculars, headlamps, telescopes, mirrors -- whatever you've got. Anything that broadens what one can observe with their own senses is going to enhance the experience. Additional items to bring include things like nets and containers for temporarily collecting various critters. These are great items to bring along, but must be used responsibly and in accord with any local laws that might apply.
Third, don't just passively observe, but be active about it! Encourage interaction (safely, of course) and the active documentation of the experience. Passive forms of documentation (e.g. photographs) are easy, but do little to make one think about the experience or to get one to pay attention to details. By "active documentation" I mean bring a notebook and measure things, count things, weigh things, describe things, identify things, and write it all down.
One of the very best ways I know to develop keen observational skills is to sketch, draw or otherwise describe subjects in a journal or field notebook. You'd be amazed at the details you need to notice when trying to draw part of an insect under a magnifying lens, or a bird at your local park. Later, encourage the use of field guides, books and online references to answer questions inspired by these experiences and notes from the excursion.
Whether you're a parent, teacher, part-time sitter, or just a friendly neighbor -- try and set aside some time today to teach someone something about our planet.