Salagou, Its a nice mince around the lake, here's a map of the marked trails there.
There is some good gnarr to be shredded too. The freeride section 2 minutes in this vid is just by Salagou. Also the bike park is in Lagamas, next to Gignac. If you like that sort of thing Google Ridekore.
Do the trails above St Jean de Fos and St Guilhem le Desert cos you get to ride medieval stuff like this, just not after 10 am in August cos its on the camino de Santiago and heaving with walkers.
Clapiers woods is worth it if you are staying in the city or east of it, but you wouldn't need a guide because most of the trails loop back onto the fire roads. On the plus side, you can be extremely lazy by catching the flowery tram out to Jacou and in a short ride, your'e in the woods.
If your'e on the south side of the city the Gardiole is the spot (big hill overlooking the med between Mtp and Sete), but you would need to be led around the good bits, loads of hidden singles.
I wouldn't suggest the FFC site at Bessilles (Montagnac) unless you are staying right next to it. Many of the way-marked routes are 60% tarmac. Did it with the family, wouldn't have bothered alone or with mates.
Plenty of road routes and stuff on this blog
The FISE is in Montpellier on the banks of the Lez, however its in May so you wont get to see Dhers, Bezanson and Co displaying their radness. Tant pis.
FYI Dog poo is now available in little black binbags in the gutter.