The beach challenge is a cross triathlon held every year at the beach in kijkduin. First you swim in the Northsea, then mountainbike through the puinduinen and on the beach to finally finish with a run through the dunes. Gruellingly tough and not for the feignt of heart, but a great experience if you do this. There is also a 3 month course for those who want to try. Check out I did this and it was the greatest ever. Only when the big day came, it was so hot that they had to shorten the course. Had a great race anyway. Also fun to come watch those idiots wear themselves out ;-)