You can do almost everything day to day on your phone. A few setup jobs still need the web or desktop app. Once they're set up there, they work on your phone.
Creating and editing smart lists: Your phone shows and uses them, but building the filters happens on the web
Creating and editing Global Filters: You can pick a filter on your phone, but not make one
Connecting calendars (Google, Outlook...): Set up Calendar Sync on the web; events then show on your phone
Turning on two-factor authentication: You can turn it off on your phone, but not on
Paying pledges and saving a card: The app opens the web page for you
Setting up your Top Mini List: Pin lists on the web; they show above your day on your phone
Setting up Email to Marvin: Find your address on the web
Redeeming rewards: Set reward points on your phone, redeem on the web
Feature settings under "Configure on desktop": Some options aren't editable on the phone yet
Looking back more than 7 days of completed tasks: The phone shows the last week
Features that aren't on mobile yet: See Which features work on mobile
The web app is at app.amazingmarvin.com. You can also open it from Account > Open the web app.