Ekg, blood all normal. Had recent onset of mild feet and ankle swelling. Unable to exercise much due to mobility issues. What else could cause. 84 yrs?

Ankle swelling. Medications taken for hypertension can, over time, cause secondary swelling of the ankles.
Edema. Edema can be from heart failure or low proteins but very commonly is caused by hydrodynamics from poor veins and lack of leg exercise. You describe this possibility in yourself and this is the likely 'what else' in you.