We love CP! Very jealous right now and wish we were planning a trip.....
Personally, contrary to other comments on here, we all love Hucks restaurant. Although it's not cheap! Some restaurants are definitely better than others though.
Towels - if you don't want to take your own, you can hire ones at the poolside. But if it's a family of 4/5 and you go every day it's cheaper to take your own! I would recommend taking a spare one each of you plan to go every day, just in case you can't get them dried out in time (but we usually go in October if we go, so can't rely on the weather to dry them).
Definitely take comfy shoes and clothing for lots of walking!
Backpack style bags for the pool, especially if you're cycling.
Take your own bikes and decent locks. Their prices to hire have gone up a ridiculous amount over the years. If you take your own bikes, take them inside if you can at night as there are lots of thefts at CP I'm told.
Oh on your last day you're supposed to strip the bedding off (someone earlier said you didn't but they definitely ask you to and personally I find it a good way of knowing I've finished checking a bedroom)!
Take a couple of games (cards, board game, etc) for a cheap night in!
If you'll be eating in the lodge, take salt, pepper, vinegar, ketchup etc. We've never taken extra cooking stuff with us, but it is very basic and I'm sure every time we've wished we remembered to take a potato masher with us or something like that!
Cheaper to take alcohol with you than to but it there, if you're staying in (obviously you can't take your own into the bars).
If young children, one of those side things you can put in a bed and a travel stair gate is useful!
The supermarket on site is great - but take any specialist/must have items with you in case they don't have it! It's also expensive, so the last few times we did a big shop beforehand and took it with us.
Have an amazing time!