This quantitative study investigates the impact of four types of discussion activities on adult learners' asynchronous online interactions. Specifically, this study aims to understand how different discussion activities incorporated into online discussion boards motivate adult learners' online participation. The four discussion activities are video-based text discussion, student-generated problem-based case discussion, self-created meme discussion, and discussion through debate. Results show that all four discussion activities, to a great extent, help motivate adult learners to actively interact with learning content, the instructor, and peers in different ways. Significantly, our study shows that participants identified student-generated problem-based case discussion as the most effective and that the debate was the least effective among the four discussion activities. Results also indicate that the instructor plays a significant role in the design, instruction, and follow-up of discussion activities. This study provides insights into using multiple discussion activities in fully asynchronous online environments for adult learners' active learning.