Go to Nuka World and walk around the outer area just outside the park boundaries - there are a few spots where mutants, random Nuka World raiders
Do few patrols through downtown Boston (just listen for the sound of vertibirds).
Glowing Sea, Gunner's Plaza, Quincy, those gunners next to Saugus Ironworks, the asylum if you haven't talked to Cabot's ghoul servant (first character I farmed combat armour pieces killing the mercs), south of Fort Hagen is filled with raiders, mutants & deathclaws.
