Are there buses around San Miguel or will I need to rent a car to really see stuff?
Public transportation in São Miguel is really not a good option... You don't have many options: you can rent a car (the best way to explore all the hidden wonders of the island, make a deal with a taxi driver to take you to see the island or go with a tour guide.
Anyway, it's a wonderful place to visit! I've been there twice and want to go back! :) almost 10 years ago