Diving is probably one of the easiest to do without using bells or NMTs. If I really need it quick to pay off some loans, I'd just burn through my tickets and try to look for scorpion or fins island or just farm bugs at night.
Yup diving is great.I like diving because I don't need to worry about tools breaking. Depending on my luck, one haul brings in anywhere between 30-65k or so.
I was thinking about doing this cause the beetle spawn rates in my town are god awful lolgoing on a trip to a mystery island past 11pm,,, removing everything but the coconut trees and beetles will span non stop!! luckily flick was on my island today and i came back and had a 320k insect haul with atlas moth being the cheapest (3k) (it takes about 2-3hrs per island tho) i always do this at night and i get about 100k++ haul per mystery island (any common map will do as long as there are atleast 3 coconut trees)