An 18-foot-tall poinsettia tree towering inside the orchid center. Quaint G-scale  trains  chugging  around the courtyards. Wacky dressed dogs parading around the lawn. It's all part of the merriment of Holidays in the Garden!  The Atlanta Botanical Garden's annual extravaganza, presented through January 2, features an expanded model train show, a train kids can ride, a Botanical Saint Nick, a holiday puppet show and - back by popular demand - the second annual Reindog Parade. At the same time, visitors can take in the ongoing exhibition Chocolate: Seed to Sweet and enjoy a number of demonstrations, classes and other tasty happenings. And the beauty of all these events, most free with Garden admission, is that they they're set against the backdrop of a festively decorated garden, adorned with traditional holiday plants and garlands.