The Westbroekpark is a great place for all sorts of activities. You can take a lovely stroll through the park, go for a run and include the park in your route, there is also a big playground for children and a large field where you can play outdoor sports, such as soccer or frisbee. Additionally, there is a Rosarium, that you can visit from June until the end of October. This is a garden where you can see different types of roses (and which is world-renowned). The flowers are truly beautiful and it is a great place to visit.
Furthermore, there is a teahouse by the waterside. You can rent a rowing boat here to explore The Hague from the water. The teahouse is accessible through the park, but during the summer also by a small boat that you can pull across the water.
In the park, there are also different events organized during the year, like “De Parade”, which is a theater/music festival, and food truck festivals. So, it is a great spot for a relaxing afternoon (especially when the weather is nice).