6 year old boys I find can vary quite a bit, some will be quite mature in their play, while others will feel more at home with activities aimed at a younger age level.
Wooden Train Track is often a winner and even the younger children can join in. Construction toys generally can work well - I care for an 8 year old who loves to build things with the large Duplo (Qudro think they are called) bricks.
Get out and about if at all possible, wrapping up warm of course. Suggest to parents that they supply welly boots. Walking in woods, park, that sort of thing. I live within a reasonable drive of Forestry Commission land, where they have an several adventure playground's located within the forest. Try finding places like that, where the little ones can walk short stretches (rest in buggy when tired) and 6 year old can let off steam. They WILL get muddy at this time of year, so suggest to parents that a change of clothes (complete change of clothes!) is useful, plus for child to wear old clothes, so that he CAN get muddy. Nothing like splashing in a stream!
Trains! Does he like trains? Is there a station near you? Can you go on a short train ride - I go from one station to the next, and back again. I live in a village, town is short car journey away, but we sometimes go by train (one stop) as the children, even the toddlers, like going on a train. It will depend on number of children you care for, would not suggest doing this with any more than 3 children.
Hope that gives you some ideas.