Coed Y Brenin is based just north of Dolgellau, this forest (Coed) offers mountain biking heaven. Ranging from routes for beginners to the more advanced, it is fantastic. Free to get in, you can either rent a bike there or take your own. If not all the party are up for cycling, there are also wonderful walks.
Take a picnic as there are lots of picnic tables or a cafe that serves both hot and cold food is on site.
There is also a visitor centre Coed y Brenin Mountain Biking Ganllwyd, Dolgellau, Gwynedd, LL40 2HZ