Ive had great success with a series of BOSU exercise:
1. Step back in a lunge position with front leg on the BOSU.
Back foot moves from 6 O’clock to 4 O’clock to 8 O’clock without coming out of the lunge.
slowly progress repetitions
2. BOSU Squat n Rock – stand on BOSU flat side up. Hold squat position while rocking side to side.
3. UP UP Down Down on BOSU – From standing position on the BOSU, drop one knee down to the BOSU. Repeat the knee. Then back to standing one leg at a time.
Altitude is a little tricky. Other than making him breathe through a coffee straw (I do not recommend ), You could progress his cardiovascular workout to include sprint intervals or tabatas (20sec. fast/10sec. recovery)
After he is fatigued you could have him stand on one leg and pick up his water bottle from different clock positions. i.e. he is standing in the middle of a clock. He would place the water bottle anywhere from 3 O’clock to 9 O’ clock and everywhere in between. Simply repeatingly picking up and putting down the water bottle while balancing on the one leg.
This will require balance and finer motor skills while fatigued yet still a relatively safe exercise.
All fun stuff. Hope this helps