You cannot miss during your stay in Alghero to go to the belvedere just before reaching Capo Caccia.
The view from there is simply breathtaking. On one side you can see the rocky Foradada island, on the other side, the wonderful quiet Porto Conte bay.
Remember to bring your photo camera with batteries fully charged, you'll surely need it.
From the belvedere you can quickly reach the top of Capo Caccia and then the decide to descend the 654 stairs to reach the Grotte di Nettuno or drive back, stopping on the way to visit the nice little Dragunara beach
You can get there by car or take Cattogno Beach Bus.