Any good places where enemy's respawn regularly?

I believe Vault 3 and anywhere Fiends are. Great place to get some sneak attack Drive-Thru good karma

Around Vault 3 the Fiends there respawn quiet often plus it gives you good karma. Around the Nuclear Test Site down south has ghouls. Black Mountain has the Super Mutants there. The scorpion gulch also is a great location with over 10 scorpions. Try cleaning up the I-15 every now and then from Deathclaws, there a few more just south of Vault 19, also alround the Gypsum Train Yard. Cazadores that respawn you can find them at Tribal Village, Brewer's Beer and around Callville bay just south of Bitter Springs. Lots of Fire Geikos around the Crescent Canyon, close to there you'll find Nightstalkers in Morning Star Cavern. That should get you busy for a while, but honestly pretty much all over the place enemies respawn.