Sometimes when I’m running I like to use the stopwatch on my Apple Watch rather than the fitness app. This means I need the app to stay active on the watch display rather than having the smartwatch default to the clock face.
If this isn’t enabled by default on your Apple Watch (and you want it to be), open the Settings app, select General, Wake Screen, then Activate on Wrist Raise. Scroll down to Resume To and switch the checked item to Last Used App.
This setting can also be enabled on the iPhone's Apple Watch app. Again, open General, then Wake Screen, and then scroll down to Activate on Wrist Raise, and switch to Resume Previous Activity.
After doing this, the Apple Watch will always default to whatever you were doing last when you last raised your wrist. This works with third party apps as well as those built-into watchOS.
Update: Thanks to Ron Bishop for pointing out the missing step!