I would say don't put the ring in food, I hear people die that way.

As for the ring, Ch-CHING!
How could she say no?

You could have the waiter bring a cake that says "will you marry me" on it. Or have a bouquet delivered to her while you're dining with a card that says the same...

Or you could get a box of chocolates and take one out and put the ring it (making it clearly visible) and then on the inside of the box on the top it asks "will you marry me."