Add specific activity to list of Flyby activities
Often times I'm interested to see two or more rides in the Flyby view, but for various reasons, the desired activities don't show up in the list (usually because the two activities didn't actually overlap or cross paths). I can imagine a button or link near the bottom of the flyby panel that allows you to add a specific activity. The "add activity" UI would probably look a lot like the "Change Activity" UI that exists today - you're presented with the opportunity to enter an activity ID or URL. It probably makes sense to require that the specified activity have a start time within 24 hours of the activity currently being viewed. Once the activity is added it simply shows up in the list with all the others.