Here are some suggestions based on my experience caring for my single brother who lives alone in an apartment building: do their laundry; drive them to doctor's appointments; when you bake cookies/squares, bring them a small plateful.

I recently went to the card shop and bought dozens of assorted greeting cards for various people's birthdays and anniversaries - my brother simply didn't have the stamina to do it. It's hard to buy those kinds of things for other people if you don't know the personalities involved, but I managed to match most of the people on his list with appropriate cards.

Another suggestion is to do some research and make phone calls to various available services (like Meals on Wheels, or the Cancer Society's volunteer drivers); in addition to that, some people might appreciate help with filling out and delivering forms that have to be filled out (this was especially true for our Mom after Dad died, and for my brother for work-related disability forms).

Even just a simple phone call some time during the day to see if they're okay (find out what times they're napping first and call when you know they're awake.) My brother sleeps all morning and rests on the couch for the rest of the day...phone calls are often his only lifeline to the outside world. I'm especially impressed by his co-workers who call him faithfully throughout the week just to say hi and let him know he's not forgotten.