Relationship is a general term that can be applied to a wide range of connections, involvements, or associations—each of us has a particular relationship with technology, for instance (some better than others!). The noun rapport is more specific: this term is used of relations between people and suggests a harmonious and sympathetic type of connection. For example, a teacher might try to establish a close rapport with students—a connection marked by mutual understanding and good communication.
Something that is fortuitous happens by chance or without known cause. In modern standard use, the term almost always carries the senses both of accident or chance and luck or fortune, as in the case of a fortuitous encounter with a former acquaintance that leads to a promising new career opportunity (a happy accident!). Beware that some stylists and editors may object to the use of fortuitous to mean anything more than "accidental." In formal writing, when your intended meaning is less about a happy accident and more about fortune's favor, the adjective fortunate may be a better option.
A bland sauce is mild and tasteless. A bland vocabulary lacks flavorful descriptors such as insipid! Insipid means without distinctive, interesting, or stimulating qualities, as in the case of remarks that are uninspired and flat, or song lyrics that are so clichéd they are utterly dull. Neither of these terms is used as a compliment, generally speaking, but insipid is the more pointed of the two, connoting at times a shallowness or lack of mental rigor.