I have used two apps. One is WorkIt which I think is great when completing challenges as you receive some extra sats. I think that @Undisciplined mentioned before something between 20-50% of invested in the challenge, fully agree with him.

The other app is sMiles . I have been using it for 8 months and have not had any issues with steps. The only downside is you must collect up to 5k sats before you can withdraw them.