While I am not advocating that people buy things for Earth Day (there's irony in buying to save the planet, after all), if you're already looking for t-shirts, canvas bags, etc., there are some really cute and funny Earth Day designs at CafePress. One that cracked me up, for instance, says "My Mom recycles more than your Mom".
To alleviate some guilt in buying, some items are offered in organic cotton. :)
Also to prepare for Earth Day, we joined the National Arbor Day Foundation this year. Dh will have plenty of trees to plant with free trees from this and also the order that he'd put in at the DNR nursery.
"To accomplish great things, we must dream as well as act." ~ Anatole France (1844 - 1924)