Todoey is the simplest way to remember things. It sits in your menubar so it is always just one click away. It blends perfectly into your macOS desktop in both dark and light theme. It also syncs using iCloud so you will be always backed up and up to date on all your Macs.
Note: The downloadable demo is version 2.0.6d; the version available for purchase on the Mac App Store is version 2.0.7.