Libraries usually have Sunday hours starting in the fall. Try larger libraries beyond your own town. Besides the public playgrounds, try elementary schools which have open playgrounds that can only be used on the weekends (beyond school hours).
Fire house - unless they are in the middle of a call, most firefighters welcome children so they feel warmly and trusting around firehouse personnel. Ours lets the kids climb in the engine, sit behind the wheel, work the lights (no sirens), try on masks and helmets, etc. Same goes for the ambulance - and that helps if ever a child should need to be transported. Take home baked goodies, a pot of chili, or veggies/dip especially for the night shift. Let your little one help in the morning, take a nap, and then go deliver it in the afternoon. You can also take goodies to the police - ours will sometimes talk to kids about the importance of seat belts and bike helmets too.
Nature areas - nothing like a hike to wear out a kid as well as develop an appreciation of nature. Check with Audubon and other land preservation groups for open sanctuaries. Most will allow a picnic if you carry out everything you carry in. Collect a few leaves for crafts, pine cones for peanut butter bird feeders, rocks that can be washed, allowed to dry and painted (great paperweights for grandparents).
Animal shelters sometimes want people to socialize with the animals - you'll have to ask if they welcome kids because many animals are under stress. Still, many need to be walked so they get a break from their pens, and if you have experience with pets, they may allow you to do this. You can also collect old towels and blankets (ask your condo neighbors) or go to garage sales for dog dishes and other accessories, and deliver these items to the shelter. If you start out with donations, they'll think very kindly of you!
Seasonal farms, apple picking, pick-your-own pumpkins? Be sure the pumpkin places don't have scary Halloween displays though.