I got from lvl 26 to 29 by grinding at Farifax ruins, *edit: Thomas Jefferson Memorial, Bethesda Ruins, and other raider and enclave hot spots. Also try wandering into large buildings that weren't part of a quest and kill everything, unlock/hack everything. Builds up experience pretty quick.
Turn up the difficulty to very hard to get a lot more experience. Although I cheat by reducing the difficulty if a Deathclaw, Albino (sp?) Scorpion, or Mutant Overlord pops up. These guys are rape on very hard. I think I'm missing a fundamental strategy to knock these guys out.
For level 29 I plan on murdering every citizen of every town to get the evil achievement, and then start over and grind...unless that good/neutral perk works at level 30.
Edit: It's too fun to be evil in this game. I just killed everyone in Tenpenny tower, and I guess I didn't do that quest with my current character so it popped and and said report to the ghouls. So I did and they thanked me, and as they turned their backs and started walking towards the tower, I mowed them down with some electric Chinese sword from the Alaska DLC.