When the beach isn't enough for a day, you're in the right place. Within half an hour's drive there are two large water parks, a dolphinarium and a zoo. Handily, they're all roughly in the same direction, towards Albufeira and Lagoa.
Zoomarine
At Guia, towards Albufeira. Dolphin and sea lion shows, a birds of prey display, and a sizeable water park with a wave pool. Allow a full day.
It's the priciest of this list, but also the most complete. Book online — it saves queueing at the gate.
The water parks: Slide & Splash and Aqualand
Slide & Splash is at Lagoa, Aqualand at Alcantarilha — both under twenty minutes away. Big slides, shallow areas for the little ones, and enough lawn to spread out on.
They're comparable; simply pick whichever is closer. In July and August arrive at opening time and you'll have the first two hours largely to yourselves.
Krazy World
An animal park near Algoz with crocodiles, snakes and a petting farm, plus mini-golf and quad bikes. Smaller and calmer than Zoomarine, and considerably cheaper.
For half a day with young children this is often more fun than the big parks — fewer queues, more doing things yourself.
One such day a week is plenty; the rest of the time the pool and the trampoline here tend to go down just as well.