This study explored appropriate speaking activities for speakers of different proficiency levels by measuring the changes in their brain blood flow with near-infrared spectroscopy (NIRS). Fifty adult speakers and learners of English of various proficiency levels participated in the study by undertaking four types of task: a single-speaker picture description, interview, and two paired interaction activities on an easy and a difficult topic. By objectively analyzing the collected data, the authors revealed which speaking activity facilitated brain activity, which speaking activity was suitable for which level of learner, and from which area of the brain it was possible to obtain activation data. In short, advanced language learners need to tackle more difficult tasks, while easy, patterned tasks such as the picture description and interview are more appropriate for elementary level learners. As for intermediate learners, their language learning is likely to be promoted through most of the activity types, except for the interactions on difficult topics, which were shown to decrease their brain blood flow.