FIL is 70 next year and wants to take us away on holiday in the summer. There will be FIL, MIL, me, DH, DD (16) and DS (13). They are pretty open about where they go, preferring it to be the right place for the DC (who they dote on). But have suggested a villa in Mallorca.
I spend a good part of my life planning and researching holidays 
so am approaching this challenge with enthusiasm, but am just wondering if I can do better?
Considerations:
MIL won't go on a cruise. Full. Stop. Though FIL would love it and it would otherwise tick most of the boxes.
DD has CFS/ME so can't travel far or do much whilst there.
It won't be a fly and flop holiday for the adults (but will be for DD) so some interest at the destination would be needed.
Needs to be warm enough for DS to swim or he'll get bored.
Although it'll be the summer holidays, somewhere that isn't too scorchio as PIL aren't used to that anymore.
Any suggestions? It would be good to come up with something that's a bit different from their annual holidays in Mallorca and Madeira.