I know this question is from 7 years ago, but thought I would offer a solution for anyone else who comes across this question in a search, like I just did.
There is no way to set only certain types of activities to sync from Garmin to Strava, but this is a usable workaround.
Set your privacy in Strava for all new activities to "Only Me." New activities in Strava will not be visible to anyone else.
After recording an activity with your Garmin that you want to be visible on Strava, edit that activity in Strava and make it visible to your Followers or Everyone, as you desire.
If you like, you can delete the other activities in your Strava account, the ones that you don't want to be visible there. Or you can leave them and they will only be visible to you.