There’s a bridleway – brand new and resurfaced – that runs from Beddgelert to the Forest and on to Rhydd Ddu. It starts close to the railway station above the maain car park. As for the forest itself, it has less than a handful of decent trails – these are short and sweet and not really worth putting a lot of effort into finding (esp going south-north) – find me on the web and email if you want more info.
There is/was supposed to be a singletrack descent from the very top of the forest – on the flanks of Trum y Ddysgl – but I have put hours into finding it and it doesn’t exist any more (or not in a recognisable, rideable format anyway).
The westernmost of the two BWs that lead out of the forest to Rhyd Ddu is worth a ride – not bad at all.