I use WidgetCal, because it has a widget that shows a whole month with events for each day. May not be the only one, but it's the one I found and started using. I used Fantastical in the past, and except for the widget, it is as good or better. Despite the flaws and lack of features, I still use Apple's stock reminders app out of laziness. Things I have done include:
Put an annual reminder of a festival or recurring event (concert, longhorn cattle drive--seriously!, tourist type stuff)
Put in monthly reminders of a book I want to read or other long-term stuff that isn't urgent, but that I want to do (park I want to visit, old friend I want to look up)
Daily reminders to file travel claims from work, entered in while I'm still on travel status, to start soon after I return.
It occurs to me I should put in a nagging reminder to find a new reminder app and start using it....