We holiday in Scotland most years with a (now) 7yo. Our favourite place though do find wellies and cagoules invaluable! Not made it to Mull but have done Skye and Oban and not had too many midges (August) so will keep everything crossed for you.
Rockpooling always a winner. We always take lots of games just in case it is really wet (still go out but good for after you've dried off ) Favourites include Yahtzee (we just take the pad and the dice so v portable), Uno, pick up sticks, mini Mastermind (which is about 30 years old but assume they still make!) and a mini version of shut the box. Oh and a pack of cards. And LOTS of books. All holidays require lots of books.