Most common things you could do on a low rate server is to grind Niffleheim. Also socked items that aren't sold by NPCs are worth something. For higher rate ones hunt monsters that drop gold:
Am Mut (0.05%), Dokebi (0.01%), Grand Peco (0.01%), Leib Olmai (0.05%), Maya Purple (1%), Golden Thief Bug (MVP) (10%), Mineral (0.02%), Enchanted Soil/Mi Gao (0.02%), Detardeurus (MVP) (40%), Plasma (Yellow) (0.01%).
Note: the rates are the standard ones; some private servers may have changed those; also remember to increase the chance by the item rate on the server you are playing (if on a private one).