The greenest valley in the Abruzzo, Lazio and Molise National Park, Val Fondillo is the main reason most visitors come to Opi. A well-marked trail system follows the Fondillo stream through dense beech forest, past springs and clearings bright with wildflowers in late spring. The main loop to the springs is around six kilometres and takes under two hours at a gentle pace, making it accessible to families and casual walkers. More ambitious hikers can continue to the Passaggio dell'Orso pass or link up with the Valle Fredda loop for a full day out.
The valley entrance is about seven kilometres southwest of Opi along the SS83. There is a car park (five euros in summer), a small visitor centre with trail maps, restrooms, and a snack bar. Picnic tables with grills are set up near the entrance. Beyond the trailhead, the forest is home to Marsican brown bears, Apennine wolves, chamois, and deer, though sightings require patience and early starts. The valley is equally good in winter, when snowshoeing and cross-country skiing take over from hiking.