Fitness trackers/mindfulness apps for wellbeing. Does anyone have experience?

My daughter's OT suggested getting a fitness tracker (eg FITBIT or similar) as a tool for encouraging her to be aware of her fitness, but also as a way of tracking her types of sleep, as she is often exhausted all day.  Having also completed recently a training in mindfulness to help combat constant anxiety we wondered about acquiring a Fitbit which also has breathing exercises as an option.  I have heard that some peole can be allergic to the straps, and also am aware that my daughter really doesn't cope with anything round her wrist for very long.  Having looked at several online, including others which can attach with a clip, I'm confused as to which direction to take  -  a simple one for £20 or something like the Fitbit Charge 2 at £113 which has more sophisticated programmes.  I'm a bit of a tech virgin so would love some feedback.  Has anyone found these useful tools to encourage personal fitness/motivation/wellbeing either for themselves or for someone they care for?