Henna ornaments can turn out very beautiful. For good results, they need to be applied with care and patience. You can make quite a ceremony out of it.
In many Middle Eastern cultures, henna ornaments on hands and feet are applied as wedding adornments. So if you use this as a symbol of commitment, you are entering an ancient and widespread tradition.
You could ask around among people of Middle Eastern or Indian origin in your neighbourhood and have it done professionally. I believe the aesthetic outcome and the joy for you both would be well worth it.
Visual inspiration: A great resource for all things henna with awesome images and tips how to do it yourself is www.hennapage.com Take a look at their collection of line art patterns.