This might not seem immediately like a Sabbath idea and you may feel more comfortable carrying out this charitable gesture on any other day, up to you, but mowing someone's lawn who is unable to, for whatever reason, is a selfless gesture perhaps God would not be upset over if carried out on the Sabbath. It is good to do good on the Sabbath. It may even open up to conversation afterwards. Alternatively, during the Sabbath you could simply make contact and organise for a time to mow their grass, or any other gardening chores, during the week ahead.