Santarém has some nice monuments, especially in the gothic style (the city is often called the Portuguese Gothic capital), and there are beautiful sights over the Tagus River and its Lezíria, which is a huge area of plains stretching for dozens of kilometers alongside the river. However, I don't think that this city deserves to be inscribed in the World Heritage List. It's an interesting place to visit at a local level, but it's not a unique site in the world - not even in Portugal. The monuments are scattered everywhere and some of them are not very well preserved. It may be worth a detour to see some churches and have a short walk in some of the nice older streets, as well as in the Portas do Sol belvedere, but I think it's not worth to be considered a World Heritage site.