If your girl's the athletic type, try a little one-on-one action on the court. Grab a basketball or some tennis rackets, and work up a sweat playing against each other.
Not into competitive sports? Try running, rollerblading, or skateboarding. If your town or city has a running path, it gives you the opportunity to see some scenery too, which makes the date a little more exciting. If you've got access to a skate park, you could teach each other some skateboarding tricks and work out new moves together.
Exercise may not sound like the most romantic date in the world, but it possibly is one of the best ways to build a stronger relationship. Working out is a natural mood booster because it pumps up your endorphins and makes you feel happier. When you exercise with someone, you associate that exercise high with that person, making your emotional bond stronger.