I visited this WHS in Spring 2024 and spent a couple of nights here, mostly to be able to visit early in the morning and around sunset when it isn't so crowded. The nearby active Fuego and Acatenango volcanoes frequently sent up huge plumes of ash in the sky every now and then, just as a reminder of why the earthquake resistant Baroque architecture was necessary here.
To cover well the main old town plus all the different tentative WHS present in the old town, including museums, most ruins and entrances, I think a long full day or two are necessary. A special mention goes to the MUNAG museum off the central plaza which IMHO should not be missed, together with the Santa Clara Convent and Church (best in morning light), the panoramic view from Cerro de la Cruz (best around sunset), the iconic Santa Catalina Arch, and the ornate facade of the La Merced Church (best before sunset).
For the foodies in our community, try out the national dish of Guatemala, Pepián, a hearty meat and vegetable stew said to be a fusion of flavours from both Mayan and Spanish culture.