I've been using a newer ad watching app called Rewardable TV. Similar to Perk, you can leave your phone running and the app plays a mixture of gifs and ads earning you 3 acorns per ad (1000 acorns = $1).
After about two weeks of leaving my 3 Moto Gs running, I cashed out $40.
I personally would only recommend this app for people with extra phones laying around. It can be pretty glitchy and will occasionally stop giving out acorns. It's only happened to me once and support fixed the issue within 24 hours.
I'm currently earning $5-$7 worth of acorns a day using my three phones but you're allowed up to 7 phones per household. I cashed out two days ago and already have enough to cash out again ($10 minimum).
To download it just search Rewardable TV in the app store. You can also start with an additional $0.50 if you sign up with my ref code (GMAIMON).
If you have any question feel free to PM me!