The goal of this clinical trial is to learn whether an optimized, multi-component mobile health intervention (ReSeT2) can reduce sedentary behavior in adolescents and young adults with acute lymphoblastic leukemia (ALL) who are receiving maintenance therapy. The participant population includes English-speaking adolescents and young adults aged 13-22 years with ALL or related lymphoid malignancies being treated at Children's Hospital Los Angeles.

The main questions it aims to answer are:

• Does the ReSeT2 intervention reduce device-measured total sedentary time (minutes/day) from baseline to Week 12 compared with usual care?
• Does the ReSeT2 intervention improve other sedentary behavior measures, feasibility and acceptability outcomes, and exploratory health outcomes such as cardiometabolic health, quality of life, and self-efficacy?

Researchers will compare participants receiving the ReSeT2 intervention with participants receiving usual care to see if the intervention leads to greater reductions in sedentary time and improvements in related health outcomes.

Participants will:

• Complete baseline and Week 12 assessments, including questionnaires, clinical measurements, laboratory tests, and 7-day accelerometer wear periods.
• Be randomized to either the ReSeT2 intervention group or usual care.
• If assigned to the intervention group:

Use a Fitbit activity tracker with inactivity prompts for 10 weeks. Receive home activity equipment and goal-based incentives. Participate in individualized coaching sessions. Participate in a peer support GroupMe chat and optional in-person group sessions.

If assigned to the usual care group, continue standard clinical care and complete study assessments.