You could turn the day at the seaside into an overnighter - stay at a nice B&B. I was going to suggest a day trip to France (ferry not train) but you're probably too far up (sorry geography's not my strong point!)
How about a messy day? Baking, finger painting - doing foot and hand prints in the garden? Picnic lunch etc.
Day trips sound good - save the money you would have spent on the holiday accommodation to go out with.
If you want to get a head start, your messy day could be to make salt dough christmas decorations with dd and then you can decorate them with glitter and stuff.
Don't forget to just chill - maybe get a video or dvd from the library, make a big bowl of popcorn - maybe allow some fizzy drinks <<Sis cowers in the corner waiting to be beaten with a stick for even mentioning giving fizzy drinks to a child on MN>>, draw the curtains and enjoy the home cinema show.
Do you fancy Cornwall? We've stayed here twice and they do free child places some weeks. The adult price is less than £250 I think. It's not the Ritz but clean and comfortable and there's a beach directly opposite and another just down the hill.