See the Ginkgo Museum and View Point. There you will find petrified wood, petroglyphs, videos, and live Ginkgo trees. All this is free but if you want to spend some cash, the gift shop offers a wide selection of beautiful things for every price range.
Fondest memory: Going across the Columbia River and seeing 'Grandfather Cuts Loose the Ponies'. A lifesize metal structures of horses in motion atop a hill.