I do a cardio pyramid on the treadmill that some people may find helpful. It’s low impact, scaleable and challenging enough that you should be sweating your ass off by the end of it.
Set the treadmill speed at a decent walking pace and leave it there. For myself that typically means a speed of 3.5 mph. Most treadmills have an incline adjustment that goes from 0 to 15 degrees. Start at zero and walk for 2 minutes. After two minutes, raise the incline two degrees and walk another two minutes then raise another two degrees and walk another two minutes.
The upside of the pyramid looks like this from an incline perspective, 0,2,4,6,8,10,12,15. Since the last incline amount is an odd number I just do a 3 degree jump at the top. After walking two minutes at a 15 degree angle (you will be working very hard at this point), I drop it back down to 12 degrees, walk two minutes, then 8 degrees, 4 degrees and back to zero, walking two minutes at each inclination. The pyramid incline numbers will look like this overall, 02,4,6,8,10,12,15,12,8,4,0
If you follow these steps your total workout will be around 24 minutes and you should be burning around 10 calories a minute on average, a good burn.
Of course there are a million ways you can scale the difficulty of this up and down by walking faster or slower, holding each inclination longer or shorter, or making the steps up and down more or less numerous.
Give this work out a shot if you are looking for a low impact way to get your heart rate going.