No place in the book tells us what Miss Maudie's favorite activity is. Therefore, we have to take an educated guess. If we do this, there is good reason to say that Miss Maudie's favorite activity is to garden.
There are three pieces of evidence that point in this direction.
First, Scout says that she and Jem were able to freely play in her yard, but they had to watch out for one thing. They must not mess with her azaleas. This shows, that Miss Maudie deeply cares about her flowers. Here is the text:
Jem and I had always enjoyed the free run of Miss Maudie’s yard if we kept out of her azaleas, but our contact with her was not clearly defined.
Second, Scout says that Miss Maudie hated staying indoors, because she loved God's creation. Here is another quote that show this: "She loved everything that grew in God’s earth, even the weeds."
Third, in To Kill a Mockingbird, there are six mention of azaleas. In five of the references, Miss Maudie is taking care of them.
In conclusion, based on these points, we can make an educated guess - Miss Maudie's favorite pastime was to garden and take care of her azaleas.