The dinosaur forest is a great place to level up. A good time to start there is when you're about level 30-40-ish. If not, the monsters around the forest give a little less EXP when combined in their largest groups. Or the desert near Maranda which gives decent EXP and a crapload of Magic Points.

The dinosaur forest contains only two types of enemies, Tyranosaurs can be pretty easily put away, they usually only do a standard attack for about 400 (I think) and occasionally they'll cast Meteor. The other type is very rare, the elusive Brachosaur (which you can steal Ribbons off .) They have good defense and aren't nitpicky when it comes to casting Ultima.