There is lots out there, last month I did the plank challenge and was impressed to get my plank up from 20s to 95s (although nowhere near the 5 mins that the plan aimed for. There is a cool app called plank a lot which has a timer and has all the days programmed in.
This month I am doing a push up challenge (as well as trying to keep on top of my planks)

I fancy setting myself an unusual challenge of when my daughter goes back to school on Monday ( another week of Easter holidays in this part of the country) of challenging myself to do a different exercise each day for a month, walking, cycling,pogo stick, hola hooping, swing ball, swimming, etc etc, might run out quite quickly, but I am getting to the "bored" stage of the diet and really need something different.

What about aiming for a certain amount if time. Like 300 mins of cardio and 90 mins if strength for the week. We can throw in certain exercises that would go towards our cardio or strength time.

How about daily challenges too. Something like 50 squats each day? Or even food challenges like no junk food or artificial or added sugars?

I was thinking along these lines too. X number of jumping jacks or squats etc.

http://well.blogs.nytimes.com/2013/05/09/the-scientific-7-minute-workout/?_php=true&_type=blogs&_r=0

I'm trying to do this every other day. Seems short enough to be doable, but makes my heart rate go up like crazy.