Yogurt is a good and inexpensive nutritious option for aging parents who enjoy it. Check labels for the best nutritional value. Also, nuts, such as almonds and pecans, can be frozen and removed a few at a time. They are very nutritionally sound. Another good options is small, individually wrapped packages of cheddar cheese which are a good source of protein. Many stores sell small packages of pre-cut apples which are excellent for senior adults who can eat them. Some frozen dinners are nutritionally sound but don't assume that because the label says something like a healthier choice, that it actually is. Check the ingredients on the back of the box.
With a little bit of pre-planning and creativity, eating alone can continue to be a nutritious experience.