Nami is not swayed by flowers like the other girls. Instead, she likes fossils and coins from the dig site. Just go there everyday, try and get one fossil and one coin, and possibly one statue even. Just give her that everyday and her heart will go up. It takes a little time, and unlike Celia and Muffy, you'll most likely propose to her the latest, but it's worth it if you want a child who is interested in drawing.