I would not want to go public, but I've seen those anonymous winner stories in the papers and they always seem to have a "Do you know who won? Call us on this number..." section under the stories.
So you'd be outed anyway, sooner or later.
I had the same plan as someone above, hide the winnings and then claim I'd won a smaller amount in a later draw. But only to very, very close family. Moving away/abroad would also be an option to keeping it secret. I might not even tell DH we'd won, but I would take him with me when I emigrated 
I wouldn't make any significant changes immediately, so people wouldn't notice.
But if I wasn't worried about people noticing, my impractical spending would be buying land to build a house on, nothing too massive, but the land would need woods and a big hill, because I'm going to carve a giant outline of my dog on the hill, like those chalk men and horses.
Then I'm going to go on the Orient Express, take my Dad on a Polar safari, see the northern lights, take in rescue donkeys, pay Kevin Bacon to do the Footloose dance with me and four of my friends, have my own crown made, and visit Scandinavia.