As the percentage of the total population living in urban regions is increasing, new challenges for the cities of the future arise. Smart Cities emerged as a solution to these challenges building on the strength of intelligent information, communication technologies and Internet of Things. In this work, we discuss the importance of semantic technologies as well as open IoT APIs for the future Smart Cities applications. Through an illustrative application, we demonstrate that both of the aforementioned technologies ease the computational burden of implementation, foster programming sustainability and create the necessary conditions so as to rapidly harness the available information and extract knowledge out of it. At the same time, the application's implementation lies in accordance with the latest IoT architectural recommendations such as Visual Programming interfaces for Service Composition, conformance with Big Data technologies and the latest IoT programming paradigms.