"What do you like to do alone and why are these solitary activities?"
Bad question, since I'm usually alone, and so practically everything I do I do alone. This is partially due to a quiet, solitary temperment, and partially due to the fact that I don't have a whole lot of friends (at least not nearby). So, sad as it is, I'm alone most of the time.
Most things I'm more comfortable doing alone, but don't mind doing in a group. A few things I much prefer to do with someone, and a few others I much prefer to do alone.
Reading is an obvious one. When I'm with someone, I like to talk or actually do something to emphasize that "hey, I'm with someone now! Better make the most of it." Reading, therefore, is a pretty much solitary activity for me, since it seems rude or at least odd to be with someone and have your nose buried in a book.
Exercise I much prefer to take alone. I don't want to be comparing myself to someone else while I'm working (though, admittedly, I can think of some exceptions).
Writing I definitely prefer to do alone, since I'm easily distracted and it takes a good deal of focus to get in "the zone."
Practicing my violin, though this is more a case where I think everyone else would like me to do it alone, since it gets pretty ugly sometimes.
Praying. This may make me a bad Catholic, but I much prefer to pray alone in private, or at least not specifically with someone. That is, I've never really enjoyed it when someone says "come on, let's pray together." I prefer to "wrestle with God" privately, rather than in a group, and I tend to experience His presence better that way. This doesn't apply to Mass, obviously.
Finally, when I'm stressed or upset I typically prefer just to retreat from everyone else and let it 'drain out' of me. Sometimes, particularly for really major problems, I like talking it out with friends, but more often I just prefer to back out and let it run it's course in private.