Like someone said, pretend your interested in another ring. Get a buddy to make a craigslist ad for a championship team ring or something. It would be difficult but a very good prank. You would want to start it like six weeks or so before your actual going to ask her. The best pranks cover little details that the target has to discover for themselves.
Anyway you could do it, your biggest hang up would be coming up with a reason for having it in your budget so long ago.
The end result would be blind siding her with an engagement in front of both of your families. I asked my wife in front of everyone, and everyone was crying except for me and my sister(that bitch).