I like the idea of a skills contest.
RB's and WR's could run a 40 yard dash.
WR's catch balls for some tricky jugs machine.
QB's have to throw the ball accurately at targets.
maybe a leaping contest for WR's
OL and DL could see who can bench press the most weight or the most reps, squats, etc.
Then the guys would have real bragging rights.