It's official—Earth Day is now a marketing opportunity gone mainstream. Once Hallmark begins selling Earth Day cards, you can say it's officially become a marketing opportunity.

Now, to Hallmark's credit, it is offering free Earth Day e-cards, as well as cards on recycled paper and a new “;Laugh Responsibly”; line made from envelope manufacturing scraps that will debut this month.

Still, let's remember that Earth Day is more about what you do—every day—rather than just that one day (Thursday) or month, for that matter.

In “;Do One Green Thing,”; published by St. Martin's Griffin ($16.99), author Mindy Pennybacker (who just moved back to Honolulu) offers dozens of simple tips on how you can be green. Pennybacker, former editor-in-chief of The Green Guide and publisher of GreenerPenny.com, says these are affordable, everyday choices that can have a monumental effect.

Tip No. 1: Free yourself from the bottled-water habit.

If just one out of 20 Americans stopped buying water in disposable bottles, we'd save 30 million pounds of plastic waste a year, she says.

Besides reducing greenhouse gas emissions, we'd save water as well. (Did you know every bottle of water sold represents three bottles of water?) Bottled water is actually less strictly regulated than tap, Pennybacker points out, while drinking tap could save you about $440 a year.

She recommends filling your own reusable bottle with tap water, but be careful of what kind you buy (she recommends stainless steel).

Tip No. 2: Choose organic and locally grown produce.

Choosing organic greatly reduces exposure to pesticides for both your body and the environment, Pennybacker says, as well as helping to preserve small farms. Buying locally cuts down on greenhouse gas emissions required to transport food long distances.

It doesn't mean everything you buy has to be organic. Pennybacker recommends buying organic for the foods you and your family eat most often. Or pick things to buy off her “;The Toxic Thirteen”; list of foods that retain pesticides: apples, carrots, celery, cherries, grapes, kale, lettuce, nectarines, peaches, pears, spinach, strawberries and sweet bell peppers.

A full ranking of 47 fruits and vegetables can be found at the Environmental Working Group's 2009 Shopper's Guide to Pesticides (http://www.foodnews.org).

Tip No. 3: When it comes to fish, eat the small fry, like sardines, rather than the big hunks, like tuna.

Pennybacker says smaller fish reproduce more quickly, in general, and are more abundant, while larger predators mature later, giving them more time to collect toxins in their fat.

For fish she lists the following in her “;Choose it”; column as being low in mercury and PCBs: farmed Arctic char, farmed catfish, clams, Pacific and Alaskan cod, farmed U.S. tilapia and Kona kampachi. Under her “;Lose It”; column, she includes Chilean sea bass, Atlantic flounder and sole, monkfish, pink Pacific snapper (except Northwest Hawaiian), bluefin tuna (Toro) and bigeye and yellowfin tuna.

Pennybacker has the right approach. Earth Day should be like New Year's—when you make resolutions and think of simple, everyday actions you can take to make your world a better place.

Pennybacker will be signing books from 2 to 4 p.m. on Sunday, May 16, at Borders in Ward Centre.

Nina Wu writes a column about environmental issues on the first Monday of every month. E-mail her at .(JavaScript must be enabled to view this email address).