I was going to say go without DH and leave baby behind with him, but you've an older Child who's away on residential
And You can't take a friend as you've probably booked one double bedroom air bnb
So It's simple fix with future in mind. Let DH have the holiday he's planned and you have yours. He will regret this if you don't pander
Leave him behind at sue bnb with his laptop , leaving him to sort out his WiFi connection, anytime he talks about his work needs you say 'well you'll have to sort that out, I'm on holiday' .
You take car or Uber to go out to all those lovely activity places. and have lunches out, go swimming, sun bathing on beach or learn surfing, or pay god primavera horse riding lessons, and go out in evening to cinema and meals, all on your own. I do it (am lone parent) and it's liberating.
I take a book with me everywhere and read at dinner, but 6/10 someone chats to me and I make new friends!
Then Make plans with other people if you meet any and dont include DH, so if he's "free later" say "oh that's a shame as I'm not now, I've plans same as you had all day with work. " Do not include DH as that undermines your point
Your DH will remember this as his crap holiday where he worked with annoyingly poor WiFi and no one brought him food or hot drinks, whilst his wife had a lovely time going out , eating out and relaxing and she wasn't available much as he tried to work during holiday.
Bet next time you book a holiday he'll take the time off.