Comment 1 for bug 497164

OmniFocus has a nice way of doing this.

For any given task, you can specify whether the tasks below it are to be done in sequence (that is, one after the other), or in parallel (that is, the order doesn't matter). This can be toggled just by clicking an icon on the parent task.

They go a step further and let you see only the bottom-level tasks that are active. Thus, for example, there's a view that would show only "Buy the gift" and any other bottom-level tasks.