3D Touch "Quick Actions" to start recording activity on Strava for iOS
iOS 9 and the iPhone 6S and 6S+ have added 3D Touch, including the new "Quick Actions" feature, where you can hard-press an app on the home-screen to bring up a small menu of quick-actions for the app
I'd like to suggest you add at the very least a "Record Activity" action, which goes as far as starting recording (not just to the record screen, but actually hits start with whatever default/last-used settings are in place), making it a 2-tap process to go from home screen to recording for most users.
That's probably the only quick action that really needs to be there. Keep it simple.
Obviously with 3D touch you could also use "Peek and Pop" for activities in the feed, segments, and well pretty much every level of navigation if you're keen. There's a lot of places where you just want to quickly check a time, distance, speed etc in an activity/segment, so that'd make things quicker too.
But mostly the quick action to start recording an activity. That'd be the perfect use of 3D touch.