Bring her somewhere she is not so familiar (you need to discover treasure, you cannot be discovering something in her vase), out of nowhere you feel something under your foot (or see a spark), get excited together with her about the discovery, dig out the ring, and turn to her and pop the question. This method involved more than just acting. To really surprise your significant other, you will need to plan well with the location and setting. The setting and environment do not matter. It is about her emotion. A swift change of two positive emotions is often touching and memorable. However, this is not recommended unless you can visualize it.