watch the waves roll in. If it's a rocky beach, there is always tide-pooling as well. Unless you are talking about somewhere tropical, I wouldn't get in the water, but it's still nice to be around the water even so.

As a point of reference, I grew up in Seattle. Even in the summer, the water is freezing off the coast of Washington. So I learned early how to enjoy the beach and the ocean without getting wet (and you can do that any time of the year).