I'd highly recommend a visit.
The lake is stunning and offers a small beach area with a safe swimming area. You can rent kayaks etc for the lake.
There are 7k, or 13k, walking/cycling paths around the lake and surrounding woodland. You can walk off the path, through the trees or stick to the path. The path is easy to walk but the first half is pebbly (so I'd advise wearing hard sole shoes). The path doesn't have lake views all around, but is beautiful and a lovely circular walk. Half way around you can take another path into the arboretum, down to a boardwalk by the lake.
The last bit of the walk is through woodland and can be a bit muddy outside of the hotter months.
Great facilities - There is ample free parking. There's a reasonably priced snack bar selling coffee, cold drinks, Ice cream and snacks. The toilets are clean.
There is a place to rent a bike. You can book horse riding in advance.