Well next year I'd maybe start looking a bit earlier!
Not sure where you are but we love the lake District. If you stay near Bowness there is a steamer, lots of little shops and ice cream stalls, bicycle hire, excellent walking which can be reached by public transport, in fact all of the lake district has good public transport connections once you get there. We stay at www.slholidays.co.uk Fallbarrow is within walking distance of Bowness, so you have at least 3 day trips which don't involve a car.
Not sure how to sort out all of your summer requirements as we have never looked at walking when we are abroad. I'd suggest James Villas and look for Leisure Resorts as they generally have kids clubs - something we look for as DS is an only and craves company.