Not to be a buzzkill, but personally, I hate surprise parties and the like, where you have to plaster a smile on your face and pretend its wonderful, when you'd rather be any where else on the planet. My advice (from a single guy so caveat emptor) is just say "Sweetie, I know you need a new car and I got you this one. Hope you like it, love you yadda yadda yadda...". Just m .02.
FQ13