A heavyhands workout without the walking comes to mind. With light dumbells in hand (depending on your fitness - 2 lb. to 5 lb. dumbells) and I guess, in your case, sitting perform a heavyhands workout by pumping the dumbells in various movements, changing the movement every minute. If you are really a beginner, you may not be able to handle even the lightest dumbells in which case your hand movements would suffice.
I use a 7-movement medley with each movement lasting one minute. Check out the heavyhands website for more detail. I believe it is www.heavyhands.com
You'll get your heart rate the highest when you are performing double overhead presses at the same time.