We did a lot of partner based dryland work... stuff where your partner would provide the resistance.  I think for the guys (a bunch of 14-18 year olds) it got the testosterone going and always became more of a competition.

Some examples:

Leg throwdowns - person doing the exercise on their back with the other standing over them... they'd raise their legs (keeping them straight) using their abs and the other person would throw their legs down to the ground.  The point was to not let your feet touch.

Lat push - person lies on their back holding their elbows above their head.  Other person leand forward with their hands on the person's elbows, using weight to provide resistance while the other person tried to pull their arms down to their chest.

Tri push - person on their knees holding their hands together in a big fist with their elbows pointing straight up next to their ears.  They tri to straighten their arms above their head while the other person used their hands to push down and provide resistance.

Calf raises on a curb with the other person on your back.

We also did a fair amount of medicine ball work, pushups, crunches, squats, squat jumps, lunges and "flutter kicking" (lying on your back with your feet a couple of inches off the ground doing a freestyle kick for intervals).

The most fun thing we did was an odd sort of exercise using boards with wheels on the ends.  We'd bring work gloves and the board would rest just above our knees, and we'd go out to a hill in the parking lot that was extremely steep.  We'd then pull ourselves up the hill using our arms... either doing a "free-style" motion or a "butterfly" motion.  Of course... we'd race up the hill.

I know that when I was swimming... anything that made an exercise a competition was more enjoyable.