When the sandhill cranes come back to the San Luis Valley, it's time for naturalist workshops at the 100,000-acre Medano-Zapata Ranch, a Nature Conservancy preserve four hours southwest of Denver. Throughout the spring, summer and early fall, three-day weekend workshops explore everything from the geology of the nearby Great Sand Dunes to the birds of the San Luis Valley. There are also weekend getaways for families, nature photography, sketching and writing seminars, and two astronomy field trips far from urban light pollution in the company of a large telescope named Hagrid. With meals, lodging with private baths, and ranch-to-field transportation provided for $450 per person ($550 for special four-day sessions), these workshops are a bargain - and they fill up quickly.