Wear a pair of shoes that won't fall off of your feet. In the summer, t-shirts and shorts is fine. Whatever you are comfortable in on the ground will also be comfortable in the air. In the colder months of spring or fall, wear long pants and a windbreaker jacket. In colder months, the temperature is about 5 degrees cooler at altitude, then there is also a wind chill.
Sunglasses are a good idea.
If you're concerned about them falling off, wear a lanyard. There is a camera mounted on the wing of the glider, so you won't need yours in the air, but a friend might want one to take pictures of you launching or landing. Tow Kitty Hawk Kites provides all of the required gear but don't forget to consider the fact that you will be getting wet. You can wear sandals on the boat, but you will probably want to kick them off for the flight. You can wear a bathing suit with or without a t-shirt and shorts over top.