To get in the party mood, I wrap up some of their toys in wrapping paper and tie a ribbon around them. I have a box of old wrapping paper and ribbon that is kept expressly for this purpose. The cake is iced party-style, with hundreds and thousands and a candle of course. If we have balloons we blow some up.
And then we have a party. The presents are torn open and the children say "what a lovely gift" in a silly voice. There's a "Happy pretend birthday" song, and the cake is pounced upon.
It's a great way to spend an afternoon, if you're at a loose end.
Postscript: I totally cracked myself up imagining doing this at work. I imagined putting a Very Important Meeting into everyone's Outlook Calender, then surprising them at the meeting with their own staplers, highlighters, and post-its wrapped in festive paper. Then a cake, and a happy pretend birthday song. Ha ha ha! Now if work really had been like that, perhaps I would have kept at it.
Postpostscript: I hope that wasn't sharing my imagination too excessively.