Another simple option…if you have minimal equipment…a simple wall sit for time. Will strengthen without adding mass. A recent study showed that isometric exercises such as wall sits and planks helped lower hypertension over time.
Too easy? Hold a couple of dumbbells. Make sure there is something like a box / step in front of you so that you don’t “collapse” all the way down due to fatigue. That won’t be too good for the knees in the long run.