candle light dinner (obviously ordering her fav food and not urs)

Roses - Red of course
Chocolates heart shaped if you plan to propose her
and a biiig greeting card which says how imp she is for you with 2 3 emotional lines straight from your heart.

I bet she will be flattered......

All the best and yes one more suggestion dont forget to have a look at her heels hehehe just in case she refuses....and i lose the bet