Bunnik is a suburb of Utrecht, mostly inhabited by people working in the Uithof (which is the academic centre of Utrecht). Except for the fortress Bunnik isn't really a place to stop.
NL Spoorwegen from Utrecht Centraal. The Bunnik "station" is just a platform. There are no taxis, no services and the city bus doesn't even stop there. Take the wijk bij Duurstede bus from Utrecht Centraal. It makes two stops in Bunnik.