When visiting Antigua you will have a multitude of hotels to choose from. However, unless you are on a budget, you should not miss the Casa Santo Domingo. It is only a few blocks away from the main square. This hotel is absolutely outstanding, a one of a kind you will not find elsewhere. This old monastery was entirely renovated and turned into a hotel decorated with the artifacts excavated during the lavish restoration. It's like sleeping in a museum. The garden with its parrots, the pool, fountains with flower petals, and the decor are magnificent. A true fairy-tale decor.
Now it's a little expensive for Guatemala (~$150), but it's really well worth it. In any case you should at least go to "visit" the hotel and the small museum. If you are looking to get married this is an ideal place too!
PS: This is where Bill Clinton and many other celebrities stayed when visiting Guatemala!