Definitely book in advance to be sure of getting a slot for anything you really want to do.
We've only been to elvedon so don't know if these are available where you are going...
Ds loves the mammal magic (lots of cute animals to pet and stroke and learn about). You do have to pay for an adult as well as a child though.
The kids show is a hit with kids (though pretty boring for adults) and is one of the cheaper activities.
Badminton or soft tennis is 'reasonable'. With a 4yr old you might find the soft tennis better. We paid £9 for an hour which included the equipment hire. Our group had kids ranging from 10-18 and they enjoyed badminton much more than outdoor tennis but we saw plenty of smaller ones trying the indoor/soft tennis hence why I'm suggesting that option.
If they have adventure golf then yes do that. At elvedon its £15 for a party of 4 iirc.
Bowling....mmmm wouldn't rave. You can do that any time at home tbh. Try something like the birds of prey experience or something different for the same money that they wouldn't be able to do so easily at home.
Hire a row boat or pedalo is reasonable price, but be ready to wear yourself out. Ds wanted me to get to the other side of the lake and back in the time slot when we did it!
Take lots of value loaves to feed the ducks (I suggest 2 per day per child - ie 1 for morning 1 for afternoon). If you haven't got many ducks/geese outside your challet go down to the lake or the village square 'river' where there will be more.
Make the most of the pool. Go for a shortish visit a couple of times a day. By 2 hours they are worn out. So perhaps aim for 1-1.5 hours with the promise to come back later. Get an inflatable ball (they will blow it up at the swim desk for free) so you can play catch.
Ds's favourite activity has always been the ariel adventure but there is a height restriction on it. It's not cheap (but worth it out of all the 'adrenaline' options imo). On our visit last week this activity was appreciated by all. You will need an adult to go round with him though as the safety harness takes some effort to 'flick' over the notches and ds couldn't manage it when he was 8.
Oh and a trip to the pancake house can be an activity in itself. Pick a pancake or an icecream and colour in pictures while you wait and get a balloon too. We had to have a separate locker for ds's balloon as we were heading to swimming from there!
And take board games like guess who and downfall and a pack of cards. Part of what I love about CP is the 'together' time with no computers or electronic games when you are having a rest time back at your challet. And that will cost you nothing if you already have the games!