We did 10 days in Denmark this year. DC's 11, 9 and 4.
We had a fab time. Rented a house 45mins from Coppenhagen for a week on a lake with private jetty, row boats, kayak and it was safe to swim in. Also had swings, slide and sandpit. We rented a car for about £20/ day.
We then had three days in a lodge near legoland with fancy swim pools, beach, trampolines etc.
We aren't sun lovers and I can't do sitting still. We had a bloomin heat wave. It reached near 30 on one day. August is just off season so prices drop quite quickly and places are much quieter.
We really enjoyed days out. Copenhagen zoo was good - you can rent little tow trucks to pull the DC and your picnic around in. The blue planet aquarium was fab - fantastic outdoor water play, crabbing lines, sandpit, artificial beach/ lake for exploration, big shark pool inside and puffins (i was pleased to see these). We visited various castles and historic places using a Copenhagen card (DC under 11 are free) it was about £60 each adult for five days from memory, but included entry to lots of places and public transport around copenhagen. Also a boat tour that took us to see the little mermaid. One thing that the Copenhagen card didn't cover was the viking boat museum and we really enjoyed that.
We found, as with all tourist type stuff, some eating out prices eye watering. But as with every country if you investigate you can cater far more cheaply. Sweets and carbonated drinks, branded yogurts like petit filou were very pricy but fruit, veg, pasta, rice, plain yogurt etc were UK comparable.
Previously in summer we've visited Iceland and that too was fab and a bit cooler. Lots of free stuff for children, geyser and gulfoss both free to visit, fantastic thermal swimming pools really child and adult friendly country. Expensive to drink but we aren't that fussed, cheapish supermarkets for food shops, reasonable nice holiday rentals many with thermal spas, watch out for expensive car hire though - cost almost as much as the accomodation.
We are restricted to school holidays and fly Easyjet booking flights as soon as they're out. Summer 2016 are just out. We've booked Prague flights at £23 each, less for the DC as they don't pay UK tax.