Do you have access to an exercise ball and/or a medicine ball?
I regularly do a routine on a medicine ball that makes the abs seize up if you do it right. 
Laying on an exercise ball (having it rest at the small of your back), do 25 crunches. Then immediately hop off the ball, hold it between your ankles, with your knees nearly straight. Now, with your hands interlaced behind your head, raise your feet and shoulders to form a "V" 25 times. Rest and repeat 2-3 times.
Then, I like to take a medicine ball (been using the 7kg, as it is wider and a little heavier, making things a little more challenging) and hold it between my knees. Now, hang from a pull-up bar and curl your knees up to your chest. Do 4 sets of as many reps as you can do.
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."In theory, there is no difference between theory and practice. But,
in practice, there is."
- Jan L.A. van de Snepscheut
"The difference between 'involvement' and 'commitment' is like
an eggs-and-ham breakfast: the chicken was 'involved' - the pig
was'committed'."
-unknown
Working on a PhD in CQB one doorway at a time.
When the wolf attacks, he will find not all who run with the flock are sheep!