When I realize I have something to do and it doesn't need to be done right away, I log it in my task inbox. Everyday I start the day by reviewing the inbox and qualifying it - is there a due date, how long will it take, is it important, etc. Then I plan my day by peaking things based on the triage. Once a week I review the entire list of tasks.
Calendars work for meetings, but for a more advanced system it doesn't.
I used to be on Wunderlist, migrated to ms to-do, the "my day" feature is awesome.
That said, I'm actually getting a lot of mileage from a new one I'm trying "Due" (iOS/mac only. unfortunately). Really lightweight reminder system with a great snooze feature at its core. Recommend giving it a shot.
My current system is two-tiered. I have a traditional to-do manager (Omnifocus) for more abstract project tracking, and a lighter weight app (Due) for reminders. Not having it all in one system, or integrated between the two is kind of a plus for me in some ways. When I get a reminder, I don't have to "load" the full context (and emotional baggage) of the overall project/task in order to make a decision. It becomes more reflexive, with the easiest options being to just do it or pick another time for the reminder.
Additonally, why can't I say, for example, that I want to work on a project for 10 hours a week, and my calendar automatically schedules it for me, to work on an hour and a half each day, based on some preferences like morning, afternoon, evening time periods? These are some of the things I want to address with my app.