One of the coolest features of watchOS 2 is custom third-party clock face complications: With custom complications, you can view upcoming flights or hotels, your next pill dosage, whether it’s going to rain in a half-hour, or how long you’ve been working out. (Better yet—you can virtually travel through time while checking this information with the flick of your watch’s Digital Crown.)
iMore scraping the bottom of the barrel for recommendations here. I wouldn’t call anything on this list a ‘must have’…
Source: The best third-party Apple Watch complications | iMore