I thought an abandoned alleyway might be a bit too grim to fit the occasion.

[oops she snatches his hand as it starts inching towards the blanket.]

Ah ah ah! It wouldn't be under there if it weren't a surprise! Hands off!

[She breaks the hug but continues holding onto him, slipping behind him and placing a hand over his eyes.]

I'll show it to you, but you can't open your eyes until I say so!