Is it possible to navigate while recording an activity?

you mean to start a workout, and run the navigate option ? Yes. Start your bike workout, then press and hold the "up - menu" button, then choose "Navigation" in the menu. You can select your route in the list then. 

Just to extrapolate one this. You first need to create the course in Garmin connect and send it to your device. Then it'll be available to use during an activity and navigate you too wherever you wanted to go.

No, he doesn't. If he can point the watch to his destination, it can calculate a turn-by-turn route to that destination.

Provided you have routable maps you can also create a 'Round-Trip-Course on the fly from within the activity.

Thanks for all the answers. If you guys know Trailforks, I'd like to load a recommended ride (basically a number of connected trails in a network of trails) and record the workout as I follow it. Bike-specific Garmin devices have Trailforks app, but for Fenix I'd have to (I'm guessing) download a Trailforks ride and upload it as a course on my watch. I hope I can figure that out, but if anyone already knows all the steps involved, I'd appreciate the walk-through.

Export it from Trailforks as a gpx then drop it onto your watch. I'd have to revisit this, but I think you can put it straight into your courses folder. You'll then see it as a course under training. You might also try importing into explore.garmin.com then synching back to your watch.