[Grail]

To me there appear to be two questions here: one is about handling hard-landscape "Next Actions" such as appointments you've made with other people, dropping the car off for a service, etc. OmniFocus handles these just fine, especially if you use the feature of OmniFocus which publishes the calendar for your iCal to subscribe to.

The other issue appears to be the "get myself to check OmniFocus". For that, the only suggestion I have is to use your daily review at the beginning of this day (or end of the previous day, depending when you do your review) to check what Next Actions you can do that day, assign start and due date/time to them, and use the "Due Soon" context to help prompt you to take action on those Next Actions.