I found this workout on none other than PBFingers website. I thought it looked like just enough of a challenge without being over-the-top hard. Try this workout if you typically get bored with your usual routine.

100 jumping jacks
90 crunches
80 squats
70 leg lifts (front, back, side, etc.)
60 jumping jacks
50 lunges (front, back, side, etc.)
40 squats
30 leg lifts (front, back, side, etc.)
20 jumping jacks
10 minute run

Pair this routine with a 20-30 minute cardio workout for an extra challenge.