Another vote for swimming pool. You’ll be spending money on it whatever you do with it, so why not spend it on refurbing what you’ve got?
I was lucky enough to have one at home when I was growing up. Couldn’t wait to get home after school when it was hot. Mates would be talking on the train home about looking forward to a cold shower, I’d be thinking how awful it would be not to have a swimming pool at home! It’s a flipping amazing thing to have.
Get the kids used to swimming when it’s cold, and you won’t need to spend much heating it. Just a bit of a bump up for parties to keep the unhardy happy! Depending on where it is? North? South?