Do you enjoy hiking? If you have access to a treadmill w/ incline, you can (somewhat) replicate it by trudging uphill.
But do NOT do what 90% of people at the gym do, and hold on to the treadmill. Just because there may be handlebars, does NOT mean you should use them. It's horrible for so many reasons, including form, posture, your lower back, leg and core strength, and of course calories. What do you hold on to when hiking outdoors? Nothing, so don't hold on at the gym.
With that important spiel out of the way, hiking uphill on the treadmill is great cardio and leg/core strengthening. You can try doing some intervals as you get stronger.