We love center parcs. It is pricey compared to say butlins, or a week in a self catered caravan in skeggy but it's all relative ; the lodges are maybe the same sort of cost as a high quality cottage for a week. The activities and the restaurants are all right there at your finger tips, but can be expensive. Possibly no more so than lots of days out though, e.g. a day at Alton towers or legoland would cost just as much if not more.
It is possible to do free things too; there is plenty of walking and swimming to be done, as well as bird hides and the possibility for some natural wild play in a relatively safe environment.
We last went in 2015 as a big family group and we did:
Owl experience £10 p.p.
Archery £30 p.p
Spa session £33 p.p
Massage £25 p.p (roughly, it was a couples thing. Individual treatments are more I think)
Evening guided nature walk £7 p.p
A couple of walks in the woods, sitting in bird hides, and using the big swimming place- all free.
The restaurants come to maybe £30 per head? Depending what you have. We did bbq's and took lots of food with us, as well as badminton kit and just played on the pavement outside the lodge, and lots of board games.
We never got the sense we were in a "forced fun happy campers" place. It was just people doing their own thing, no one is really interested in you and your group! We find it really chilled out. It's a bit like eurocamp but with more stuff to do and without a fecking tent, for me.