My site uses an Activity Stream as the primary home page.  We have a site defined default stream that members can override (which updates the link to the far right of the bread crumbing on the page as an effective "quick link" to it). 

Is there any way a member can define their default Activity Stream and have the site's default homepage land there for them as opposed to bookmarking or doing a multi-step process of visiting the default homepage and then clicking their default stream on the top right?

Unfortunately, only the All Activity Stream can be set as the "home page." Currently, this cannot be configured to anything else.