Located in the Taurito Valley just east of Puerto Rico, Lago Taurito is almost always sunny and has an open door policy that lets you bring in your own food and pop out to the supermarket just by the entrance.
It also has a good minigolf, a cafe that offers a buffet, and mini slides for small children.
Entrance fees are reasonable at 17 euros per adult and five euros for kids between five and 12. It's open every day of the year.