OK - so it's hardly part of the 'Picturesque Algarve' countryside, but it's a lot of fun! It's a great day out, and you can buy discount tickets in the village or the Intermarche supermarket which also means you can get in when it opens at 10am without having to queue
Slide and Splash
There are several water parks around, but by far the best is Slide and Splash at Lagoa.
(Click on a thumbnail to show the image larger above.)