My mom did pull of a great joke one year, giving me mashed potatoes and gravy disguised as vanilla ice cream with chocolate sauce. It worked really well. Either she's a great cook (which she is) or I am more gullible than I'd like to admit...she's a great cook, we'll leave it at that!
That said I think I'm actually going to try to pull something off this year. I really am. I found a plethera ("do you think I have a plethera?") of ideas here at Family Fun. They always have good ideas.
Try one of the ideas on your family and let me know how it goes...I'll report back later.
Oh, and for the more adventurous of you, my dear readers...here's one that is sure to shock people. Take some ice cubes, dip them in chocolate, just like you would if you were making chocolates. The chocolate will immediately harden around the frozen ice, a water-tight seal for the water. Allow the chocolates to sit until the ice cubes have had a chance to melt, then serve.
Trust me on this one, I know it works as a great joke... :)