Giant Sequoias [wiki] (Sequoiadendron giganteum), which only grow in Sierra Nevada, California, are the world's biggest trees (in terms of volume). The biggest is General Sherman [wiki] in the Sequoia National Park - one behemoth of a tree at 275 feet m)
Redwood Forest, California I've been there, me my mom and my dad entered a hollow tree and there was room inside of that tree for 10 more people. I measured how many steps it took to get around a tree an it took 32 steps. The place is just beautiful.
The Dyerville Giant, one of the largest trees in the world. When this massive tree crashed to the ground in you could hear it for miles around. It sounded like a train wreck. ("Avenue of the Giants," Humboldt Redwoods State Park, California)
During a summer vacation to central California, one of the caches I found was inside an olive tree similar to this one. Living on the east coast, I had never seen an olive tree before and I thought they looked so cool.