If you want activities book them online before you go, but with your DC I would just book them into the creche/supervised kids club for one session so you can chill.
There are lots of free activities for younger ones, playgrounds, walking, cycling, wildlifevwatching (red sqirrels, deer, ducks, rabbits, birds etc), small beach by the lake and of course swimming.
If yours are anything like mine prepare to swim twice a day. We went first thing as kids pool opens before the main pool, and around 5pm as this is another quiet time. There are lots of pools, slides etc to explore, it's nice swimming outside in really warm water!
I would take lots of easy food so you dont have to shop, cook or eat out unless you want to. The food is all a bit overpriced and naff imo.
Also worth taking bird feeders and nuts for squirrels to attrack them to your back door.
Take bikes/trikes/scooters as it is fab having solid tracks with no traffic for DC to ride on. Locks are essential as everywhere you go has big bike parks and you cant take them inside.