ATMs are always a great idea if you have the bank roll to purchase the machines AND load them up with cash. I'd say $3,000 in each machine you buy would work at a slow location.
You can have this running passively pretty much as soon as you launch. Just get a few bars or restaurants to let you put your machines in and then check them every week to see if they need to be filled.
Some machines even let you go online to check their balance lol. I'm not sure if they will send you an email alert or not if the balance is low, but that would be pretty amazing if they did
Some of the machines are expensive, around $6,000. I think you can rent machines too.