Love the Farmville/Candy Crush insight. How do you handle the virtual pet mechanic without violating your 'no punishment' rule? For instance, if a user gets sick and can't walk for a week, what happens to the pet? How do you encourage them to return without inducing the guilt of a broken streak?
So, we don't do any of the typical "stick" options. Your pet won't get sick, it won't die, it won't regress in levels. Within a week, it gets a little sad if you're behind on your goal, but there are no longer term effects of this. If you miss a week, your pet just hangs around and when you come back, it'll start progressing again.
If you're sick / on holiday / life gets in the way for a while, our adaptive goals kick in, and lower the difficulty. So when you do come back, your goals and challenges are at a nice achievable level, so you get a nice pump of progress straight away. The idea is that returning should feel good, not guilty.
We're also heavily against streaks, daily or otherwise. We promote rest, and integration of fitness into everyday life, so we actively encourage days when you don't hit your target. And, we think the whole "streak" rhetoric has demonstrably failed, it causes more emotional stress than any gain it provides.
