Josh is off by two train stations... Sakuragicho is okay, but the number one dating spot in all of the Kanto area is Ishikawacho.... the park near the Yokohama New Grand Hotel is famous for its beautiful scenery, quiet places to walk, and wonderful restaurants.
Of course, why wait until summer? Take your girlfriend up to Niigata for the skiing (two hours by train from Tokyo)... it's awesome.