What else can I do, in addition to feeding a Muslim for each day of Ramadan, if I have a stomach problem that prevents me from fasting?

If the illness that prevents fasting is not expected to be cured, then feeding one poor person for each day is correct, and nothing else. However, if the illness is expected to be cured, then the missed days must be made up after recovery, and feeding is not permissible when one is able to make up the fast.