The permanent use of smartphones impacts the automotive environment. People tend to use their smartphone's Internet capabilities manually while driving, which endangers the driver's safety. Therefore, an intuitive in-car speech interface to the Internet is crucial in order to reduce driver distraction. Before developing an in-car speech dialog system to a new domain, you have to examine which speech-based human-machine interface concept is the most intuitive. This work in progress report describes the design of various human-machine interface concepts which include speech as main input and output modality. These concepts are based on two different dialog strategies: a command-based and a conversational speech dialog. Different graphical user interfaces, one including an avatar, have been designed in order to best support the speech dialog strategies and to raise the level of naturalness in the interaction. For each human-machine interface concept a prototype which allows for an online hotel booking has been developed. These prototypes will be evaluated in driving simulator experiments on usability and driving performance.