Well here is what I know about the Daily Challenges.
1. You get one new challenge at 1 a.m. PST each day.
2. You can have up to 3 challenges at a time. You can store daily challenges in the lead up to events.
3. By pushing the x in the upper right corner of a task you can exchange it for a new random task once every 24 hours. This is 24 hours from the last time you exchanged a task.
4. During Big Events you can get whatever currency the event is using for the daily challenges.
5. When not in Big Events you can get money, Springfield Heights Currency (Furniture, yoga mats, coffee, etc...), and donuts from the challenges. A lot of people exchange daily challenges to try to get new ones that offer donuts.
Things I can not confirm is whether or not they still do crafting currency for Daily Challenges during events. I don't remember the last time I saw them actually do that. As for exchanging a task after it's completed I am not sure about that either. I will try to check that out tomorrow.
