Fitbit is something everybody bought last Christmas because they already own a phone, iPad etc. and they were out of ideas. It's stupid and your kid is just going to lose it anyway.

What about a simple pedometer? My 9 & 7 year old have a blast monitoring "steps" they've taken. They also experiment with them when riding around on their bicycles. They also try to get to a certain number each day. They've had them for about a year, and they're currently using them non-stop but will probably put them away by July!