My agenda is more like a switchable list of possible tasks.
I have it divided in two groups of tasks: hard and soft. Any of the tasks can have dates or times (be it schedule, deadline, or simply reference of "good for" that date), but the general rule is to avoid using dates and times.
Hard group is for things that don't repeat daily or that there is a penalty for missing it.
Soft group is for things that repeat daily or that I can make up for the days missed in the next successful completion without prejudice of losing "uniqueness" from the previous misses.
From this list I cycle through items by doing one from each group every 30min, starting from the group of hard tasks. Doing breaks of 5min for each task, and of 15min for each four tasks.
Also, I avoid picking tasks that have an action in common to the one I'm currently doing. For example, if I must write an important message now, then the next task musn't involve writing.