Interlaken is a better base; Grindelwald is too congested with walkers and, perhaps, too high up (interlaken has the benefit of being right at the bottom of the valley thus maximising every descent!)
Try the singeltrack from Schynige Platte down to Wilderswil, and another good one is from Morningberghorn to Darligen. Both are mind blowing singletrack, but take a bit of sweat to get to the top.
Also, have you seen the Kompass digital maps? very good for finding singletrack, and you can buy them on amazon.de from the UK.
EDIT: if you're looking at the Lenzerheide region, Davos has some fantastic trails, and is a lot cheaper in the summer than winter (dont know about campsites though)