Sincerely tell her how beautiful she is at any random moment. She could be dirty in the garden, or doing dishes, wherever - stop, be amazed, and say she's beautiful.
I would say the best romantic moments that I have experienced were not planned. They were just great perfect unplanned moments. I would say just go out or stay at home and try to start it out romantic and maybe the rest of the evening or day will continue on its own to be romantic.
