if you can swim, there's an exercise some swimmers do,
called "pulls". pretty simple, just swimming freestyle with
your arms, and a float between your legs.
that's version #1. version #2 - do pulls, without the float.
it is some serious triceps torture.
what will happen is, as you get tired & slow down, possibly
in the first 25 yards, your body will lean at an incline, and
you will have to work harder. you will feel like you're going
really slow. but your triceps will get a heck of a workout.
do that 2 lengths of a pool at a time. repeat 4 times, if
you can. if you don't swim regularly, you will feel it in your
triceps for days.
that part should take 20 minutes. to make use of the
workout time, grab a kickboard & kick a few laps. this
will work your lower body, from your abs to your knees.