You don't have to be religious to enjoy the tranquil atmosphere of Andohalo Cathedral. Check when the next service starts or when it's open for general viewing.
You'll soon see why this Antananarivo principal church is a hit with travelers. If you're searching for other things to keep you busy, Analakely Market, Rova, and Independence Avenue are in this area as well. They won't take you long to get to from Andohalo Cathedral.