Now you're in your mid-twenties, level-wise. Good job. Now you're noticing an increase in exp tnl and not enemy exp. Well, you could try Relidia if you're suicidal. Relidians give 13308 exp each and take a while to kill (you'd actually be better off against Imperial mobs), but the Patrolwomen drop a Steel Flail and an Elven Cloak. The former sells for roughly 2200 gil, and the latter for 9000 gil, so it's okay money. Really, though, you ought to just kill Wirey Dragons for fast levelling, but if you like a challenge, Relidia presents that. Once those drop off, the U'Tama Waterfall can pick up the slack, as can Serpent Trench or the Mysidian overworld. However, don't expect it to be "mobs fall down at your feet" easy.

Once you hit level 35, you can probably join a second-tier class (if not, level up a few more times until your stats meet the requirements), so make sure you either have Black Wizard or Monk. The former gets the tier-2 elemental spells (which will miss less than you've probably noticed tier-1 spells do), and the latter gets Suplex. Suplex deals a lot more damage in the short run, but it takes a lot of clevelling to get. In any case, Relidia will probably be your levelling home for the next 30 or so levels. You can also level in New Hauswald if you can find it. Mysidia offers decent experience if you can stand the Zuu birds. At level 45, you should do the Find Thamasa quest.

Once you're in your 50s or 60s, you might consider the Phoenix Caves. There's a quest in there so I can't say how to get in, but the mobs are good for experience if you abuse Suplex or Pool (I'll leave this one here for the nostalgia factor... it's Blizzard now). Make sure you have either Clarify, or a Steel Close Helm before fighting Phases, though, as they confuse you. Until higher levels (and even then) you won't want to fight multiple mobs at the same time.

At 70, you probably won't meet the requirements for a third-tier class (you'd need a few statbonuses for stats with a racial 3, or literally every level with a racial 2), however if you do, Black Archmage deals a lot of damage, as does Sensei at higher class levels. The choice is, however, a matter of taste. Once you hit level 80, you can go into the Eastern Kingdom and fight Leapers and, if you can survive going that far in, Onracian Guards and Troopers. You're set with those if for some reason you feel like "high risk, high reward" levelling. Otherwise, go for Daryl's Tomb with either a Shadow Ring or the Sneak or Hunt skills, or Ebot's Rock if you're feeling particularly daring.