It depends on what you fancy doing. There’s a couple of downhill trails in the area, but there are also XC epics. The easiest to navigate is this one here. The best place from Bethesda to start is from the A5 opposite the camp site (I can also scan a better route map, albeit slight shorter, if that helps). If you’re also looking for family oriented routes the North Wales path is nearby and there’s a cycle path that goes right through the Ogwen valley. Just ask if you want more detail.
Edit: if the route suggested above is more than what you had in mind, there are milder runs around. A loop around Moel y Ci and the slate mines above Llanberis is fun, although involves a fair bit of tarmac.