My work takes me to the western coast states often. The two places I have been to alone that I really wished I'd had someone to share it with were:

Multnomah Falls, Oregon
Santa Cruz, California.

There are other places in my territory that I know would be great with someone special (like numerous wineries, Yosemite National Park), but I've been avoiding them as I really would like to visit them for the first time with someone special.

There are many beautiful places in this country, but if need be, I can make the picnic table on my deck the most romantic place in the world.