Encouraging patients with cardiovascular disease to be more active is one of the critical challenges in conventional rehabilitation programs. Rehabilitation programs are mostly offered at specific times and locations (e.g., clinical facilities or in-home exercise programs), which are the root causes of participation barriers. We envision that use of mobile and wearable technologies can overcome such spatial/temporal restrictions and offer novel opportunities for promoting physical activities in their everyday lives. In this work, we introduce BeActive, a mobile-based intervention platform that delivers just-in-time intervention messages to help patients to be more active in their daily lives. The key innovation of BeActive is to leverage fine-grained tracking of user behaviors to deliver just-in-time feedback and to incentivize health behavior maintenance via micro financial rewards. To show the feasibility of BeActive, we conducted a preliminary study for seven days (n=5), and presented several design improvements for just-in-time health intervention.