The main town is Vila Baleira, which is basically a small but very neat village. In Vila Baleria there is also the Columbus Museum (was the house of Christopher Columbus), the explorer had arrived in Madeira in 1478 and went to live on the neighbouring island after marrying the governor's daughter. In the museum you can see some rare maps, paintings and engravings.
North of Vila Baleira on a hillside there is the lookout point at Portela, which offers on island wide panorama and some restored windmills.
On Porto Santo I stayed at the Hotel Vila Baleira, with it's nearby Thalasso Therapy Center.
I really enjoyed my stay on the island, I had 6 wellness days included and on the last day I rented a car to explore the complete island, which is quickly done. If you are looking for parties and high life, than please stay away from Porto Santo. But if you are looking for a peaceful place and wanna have a long sandy beach almost for yourself than Porto Santo it is! ;-)