There are several cool trails to mountain bike ride in the St. Helens monument area. Check out: Green River Trail (#213/7.5 mi.), Goat Mountain (#217/8.6 mi.), Boundary Trail (#1/20.2 mi.), Butte Camp Trail (#238A/3.7 mi.), Loowit Trail (#216/28 mi.).
Of course, the most popular area around Mount St. Helens is the Plains of Abraham area with its windswept pumice moonscape with the strange trees that were striped from the eruption.
Overall, Mount St. Helens National Monument is the 26th most popular mountain bike trail of all 59 mountain biking rides in Washington.