I have arthritis in my knees & I get migraines easily - help with exercise ideas?

I can not run due to the arthritis in my knees, swimming is out of the question due to financial reasons - so I walk, but that gets boring. So any ideas for other cardio would be great. Also - need to work on abs but get migraines whenever I do sit-ups. Yes I have had aerobic instructors watch me do them & they say that I am doing them right & not pulling on my neck, etc... but still get migraines anyway. Also get migraines with strenuous cardio workouts. I really want to lose weight, but it just seems that there are obstacles around every corner. So any suggestions/advice would be greatly appreciated. Thanks!!!