Fitness+ gets you exercise video classes
The Activity app tracks your activity, in various exercise categories. And even if you do not enable an exercise activity, the Activity app will still attempt to track your movement, some exercise, and your standing minutes (which amounts to your watch arm moving as if you were walking, even if you were not), the stairs you climbed (estimated), etc... You can also track your heart rate if you like.
As a passive exercise monitor, I've found I do more just because the Apple Watch is tracking things. You are older than me, but not enough to be my parent (that should give you an approx age range).
While I find the Activity tracking nice, my big likes about the Apple Watch are alarms, alerts, notifications on my wrist, answer (or reject junk) calls on my wrist, the weather on my wrist (I do that a lot), iMessage with my Wife on my wrist, "Hey Siri, 15 minute timer", and it tell the time and date (which I can get a dozen other ways in my home).