Ils very generously want to take my family and sil's family away on holiday - we're looking for somewhere where we can all do separate stuff in the day, as we're a wide age range (Dc ages range from 13 to 4), and so we don't all kill each other, but then get together at mealtimes.
They previously took us to Disneyworld, on a similar trip, which was a big hit - something similar would work, I'm thinking of a resort in somewhere like Greece/Spain that offers activities for those who want them but where mil could snooze by the pool. A villa doesn't work as mil would feel compelled to cook and clean.
I bet your PsIL won't mind another cruise one bit! Haven't travelled with children for years so can't recommend from personal experience, but you may as well go bells and whistles if you want all ages to enjoy it. There will be golf nets, climbing walls, flumes, open air cinemas, you name it. For those who like a bit of tranquility (i.e. me) there is always a cocktail bar, some high end premium dining options as well as the usual variety of eateries. You will all be waited on hand and foot and will not want to come home. Ooh, jealous now!
My father very generously took his family for a holiday for his 70th and we went on a Neilson to Turkey. Lots of activities for the kids and those adults who like to be active. Kids clubs to entertain kids as well. Lovely pools for those who like a more sedentary holiday and history and culture trips for those of a more cerebral inclination.