I think that hobbies can classified into three broad categories: solitary hobbies; group hobbies; and hobbies that can be either.
For example, solitary hobbies are:
-model boat building
Group hobbies are:
And, hobbies that can be either solo or group are:
-visiting the opera, seeing concerts and plays, going to movies
-weightlifting (i.e. you could go with a friend, or by yourself)
-going to sporting events
-playing musical instruments
These aren't exhaustive lists, of course. But the ones that came to me were either solitary, or "flexible": not too many were inherently "group".
I found it interesting that all of my major and minor interests were either "solo" or "solo/group" -- which I practice solo.
Specifically woodworking, songwriting, recording, playing music, gardening -- I do 'em by myself.