for a complete suprise:
you could do like this, steam open a box of cracker jacks, and put the wring at the top with a note. then glue it back together. gave it to her when they were watching a movie, and say can you open this i'm gonna go get a few drinks. then watch from the kitchen as she opens the package, and walk in just as she realizes.
downside ... sticky engagement wring.
For a more romantic thing:
you could go online and find a nice hotel in a city or small town (whichever you prefer) close to you(hour or two). take a trip up there a few days before and make arangements to have the room filed with flowers/chocolate/wine/candels ect.
tell her you want to go on a little road trip on a saterday morning and comletely suprise her with the room, dinner ect. and if at any point when your driving she asks where your going, tell her your parents house, and just make fun of her till she stops asking.
I could come up with somemore if you want ... I'm a romantic cause i love bitchs
