Built between 1652-53, Taiyuin-byo was constructed a short time after the completion of the grand and ostentatious Tosho-gu. While Tosho-gu is the mausoleum of the first Tokugawa shogun, Ieyasu (1543-1616), Taiyuin houses the remains of the third shogun, Iemitsu (1604-1651). In comparison, Taiyuin-byo is far less grand. There are a few views as to why this is, but many believe he either was afraid of Ieyasu even after his death, or that he greatly admired him to the point that he didn't want to out-do him. In either case, it's definitely something to check out if going to Nikko.
Taiyuin-byo. Nikko, Tochigi.