On a hot day, Belgian beaches are full. The sand is fine and clean, the water is somewhat warm, and there are plenty of activities besides beach games. There are go-karts set up for kids with an actual track. It's a lot of fun for kids for 3 euro/half hour.
The pier is beautiful—you can walk along it or rest on one of the many benches and just watch the world go by.
Inside there is an exhibit called STORMS. It teaches you about storms, tornadoes, and hurricanes. You can experience a tornado or get inside a chamber and experience a hurricane with 100km/h winds. Pretty neat. The only downside is that all the information is in French and Flemish, and we are rusty with both languages. Along the boardwalk there are many, many small restaurants and terraces serving delicious moules frites (mussels and fries) and other delicious food.
All in all, not a bad way to spend your day.