Add my votes for most of the Barks and Rota ones you mention. One reason I particularly remember The Swimming Pool Attendant is the cover of the Journal de Mickey in which it appeared, an expansion of a splash panel from the story. The woman on the beach in the foreground is reading this very issue of Journal de Mickey, making this one of the "infinite regress" joke covers, of which I am fond.
Teresa Radice & Stefano Turconi's excellent l'isola senza prezzo starts with Donald overheated in a scorcher of a Duckburg summer, and the Greek island he goes to with Fethry and Scrooge is a fine vacation spot, in appearance and in spirit.
My sentimental favorite is Terry LaBan's The Great White Whale--because the Ducks (Donald and HDL) are visiting Cape Cod in Massachusetts, and I'm a New Englander. I read it in French, so the grammatical gender of the word for "whale" in French gave the descendant of Moby Dick feminine pronouns. I'm sure I would be disappointed if it were printed in English and the whale was male! But in any case, the summer vacation vibe is very close to the summer vacation trips of my own childhood, and I like the happy ending for the descendants of Captain Ahab and Moby Dick.
For ten days from now, the summer solstice story is Gail Renard's Brig-a-Dog, an homage to Brigadoon, which was my favorite movie in my youth. It's satisfying to have a version of Brigadoon in Scrooge's Scotland.
I'm assuming that Cornelius Coot Day takes place in summer, so David Gerstein/Vicar's Pioneer Daze also goes on my list. I love it for adding a holiday to the Duckburg ritual year...plus it's funny, with great Vicar crowd-chaos scenes.
For the last hurrah of summer vacation before school starts again, there's the McGreals/Vicar's The Secret of Goblin Valley. The ending is quite funny for an American who grew up when the first thing you had to do at the start of the school year in September was to write an essay on "what I did during my summer vacation."
I'd also add some more stories that take place on the beach, such as Barks' Rival Beachcombers and Beach Boy, or the various short stories where the Ducks build sand castles, like Tom Anderson/Beatriz Bolster's Sand Castles.
Are there any good stories featuring HDL at Junior Woodchucks summer camp?