Could you ask her parents long, long, loooooong in advance and not tell them when you're planning to propose? That way they won't spoil the surprise at the exact time. I assume she and her family are expecting you two to get married, right? So you won't really be giving anything away.
I like the idea of the flowers. I think you should choose flowers whose initials spell out "marry me"--a mum or marigold, an aster, a rose, a rose, a (hm, what begins with Y? gardeners, help!), and so on. Okay, maybe not--maybe that would be too hard.
The symphony idea would embarrass me so much that I might say no, if I was the person being proposed to. However, I assume you know Liz well enough to know whether SHE'D enjoy it.