I like Karin’s idea of step counting. So that everyone has a chance of winning something, you could have a raffle each week. Maybe a participant gets 3 tickets in the raffle if they do 10,000 or more steps every day. 2 tickets if they do at least 50,000 steps in the week (10,000 for 5 days or about 7,500 per day). 1 ticket if they keep track, no matter how many steps they do, because observing and measuring their behavior is a big start for some people.
Another thing I have done with my small-group members is to get them to cross training by challenging them to do at least one of each of three types of workout in during the week – a strength, a cardio, and a yoga/stretch/roll/recovery.