I learned that the Raider bridge north of Vault 101 is a great place to hit over & over, because Raiders respawn there every few days. I use that early in the game to build up XP, loot, and caps.

There is also the outside of the Springvale Elementary School, though only three raiders will spawn there.

I'd like to come up with a list of places to hit every few days, starting with those you would likely find near the biginning of the game, and branching out to include locals you discover later.