If you live in or near Tokyo and are trying to come up with some Valentine date ideas, let me recommend the Sagamiko Illumillion at Pleasure Forest. The organizers have created a fantasy world out of 3 million LED lights. The park has been separated into different themes, including a section that models our solar system.
There are also amusement park rides to enjoy, including a Ferris wheel lit up like Mt. Fuji. You have to take a ski lift over the lit fields to get there, which is almost worth it in and of itself. Other attractions include the largest mirror maze in Japan, a snow field for the kids, and a variety of food stalls.
The illumination is set to continue until early April, so you have plenty of time to visit. But be sure to dress warmly when you do, because it gets darn cold atop that mountian!