I am surprised no one has answered, so I will give my comments.
I went to Longleat Center Parc about 6 years ago so my comments are based on that, but it may have changed.
Chalets are placed inside lovely woodlands, with plenty of space between each Chalet. The site is huge.
You can take you own bike to cycle through the woodlands, or hire a bike. You can also of course do lots of walks.
All the cars have to leave the public site and park in designated car parks, so the roads and woodlands are very safe to cycle or walk round.
At the time we went only the swimming was free, although it was an amazing swimming complex, part indoors and outdoors, with heated water.
There were lots of things you could do on site (sport, bowling, massage etc) but they tended to be very expensive so it is easy to get through a lot of money.
Overall we enjoyed it, but be prepared to get through a fair bit of money.