Perhaps, delay the reveal until the dog is at peak what-the-hell-is-going-on mode. And then, bang.
Well, as this guy so powerfully demonstrates, all you need is a blanket.
In a video, posted on The Chive, a man returns from vacation and quickly hides under a blanket. His faithful friend doesn't realize he's back, but he knows something's up.
First, he hears his owner whistle. The dog wastes not a second in appearing, massive green toy in mouth.
Then he sees the lump under the blanket. He sagely drops his toy. This may be a real situation, requiring him to be a serious dog.
He creeps ever-so-tentatively up to the blanket, tail at half-mast.
Could it be ... ?
His owner whistles again.
Dadd ... ?
And then, when neither human nor dog can contain himself any longer, a final clear whistle.