My girlfriend (a med student in the USAF) did her own "100 pushup" workout for her fitness test.
1 pushup, rest briefly
2 pushups, rest (briefly = about 5-10 seconds)
3...
4...
etc. Once you do 10, you come back down. It equals 100 pushups, done over about 5 minutes.
I did it with her once. I usually do 30 on my own after I run, and maybe 1-2 times/daily. It burned...
That she could do it (all 5'3" and 120#) was embaressing. But, might be an option for others?
For those that think pushups are too repetitive on the same muscle groups, you can make them inclines or declines, by either having your hands or feet on a stair (or 2). Makes the weight shift, so other aspects of the muscle groups get worked out.