What is Miss. Maudie's favorite pastime?I tried looking it up in the book but I couldn't find anything.

I think that you should look for this answer in Chapter
5.  This is the first time that we meet her in the book and the author sort of
introduces us to Miss Maudie and tells us about her.  The author does not come right out
and say "her favorite pastime was..." but you can infer what her favorite pastime is
from what it says in the chapter.

If you look in there, it
talks about how much time she spends on her flowers.  It also tells about how she treats
fighting weeds as if it were the "Second Battle of the Marne" ( a big battle in WWI). 
From that, I would infer that her favorite pastime is gardening and/or taking care of
her yard.