Fantasia Lagoon is a rooftop water park above The Mall Bangkapi, with pools, a lazy river, and slides aimed mostly at families. It is not one of Bangkok's grand sights, but it can be a good break from heat and traffic if you are staying on the eastern side of the city.
The appeal is practical: cheaper and easier-going than the larger theme parks, with enough water play to keep children busy for a few hours.