Then, they can take a 90-second rest before repeating this for 15-20 minutes. If you have a flight of stairs in your gym, your client can walk/run up and down the stairs as many times as they can within 30 seconds. Have your client carry a sandbag on their shoulders while performing a stair climb. If you have a flight of stairs in your gym you can use that otherwise, I suggest using a Stair Master machine or something similar.
Stair-climbing might feel like this in the moment, but your clients will thank you for it when they start seeing quick results!