There are activity tracker apps. On the iPhone it is "built in" on the Health app. A phone is not particularly cheap though......
The cheapest is a step counter/pedometer. Here is one that has a 30 day memory for $25 - you can manually log the data when you feel able too, say once a week into a free app such as Google Sheets.
Amazon product ASIN B078HWTK5PCode:https://www.amazon.com/gp/product/B078HWTK5P/
More brainstorming [I'm a member of this group https://europeanmecoalition.com/]:
- does this study show that questionnaires overestimated activity?
- does this mean that activity monitors would, in general, make it more difficult to (wrongly) argue that someone has recovered (post GET & CBT) and therefore shouldn't get social security assistance/benefits?