I'm a corny kind of guy. The way I see it is that she has to be both surprised, and open to the idea of my proposing. Here's the plan, and I know this sounds kind of mean, but hear me out.
My gf, Leeza, likes everything to go right. So I thought, what if she had a day where things went wrong, and then when I popped the question she'd be really happy that something went right.
She loves her car. It's a Honda and never breaks down. When she goes out to get in her car to go to work, she has a flat. I've already hidden her AAA card, so that creates havoc. I swoop in, and drive her to work promising to have her tire fixed.
She's going to be late - but her boss is in on it so no worries. I take a wrong turn and end up on the freeway. I know she'll be yelling at me, freaking that she's late. I keep driving. She calls her boss but he puts her on hold. I keep driving, and in about 15 minutes we are out of the city and in front of her elementary school. She took me there once to show me and always talks about her favorite teacher.
Before she can wonder why we're there, I tell her I need to pee. I park and hurry in. I know she'll get out, and when she comes in the building and walks down the hall, I find her. I suggest we walk around. I ask her questions about the school, about her as a little girl, etc. At my urging, we go into a classroom. Her favorite teacher's classroom (who is in on it). She is going to be embarrassed, but happy to see her teacher. The teacher is going to say we are lucky, today is show and tell. We take our seats. When the teacher calls on me, I get up, drop on one knee, pull out the ring and ask her to marry me.