Aquarena Mogyorod near Budapest
Aquarena is an amazing slide and water park next to the Hungaroring, near Budapest. It has 21 slides, jacuzzi, swimming pools, but it's main attraction is the cave lake, featuring the Island of Capri. Beside the water adventures you can play volleyball, football and badminton. Tickets are about 5000 HUF.
For ideas on accommodation in the area click "book now". You don't have to pay in advance but when you arrive. Free cancellation on most rooms.