Plaza with a railway museum (plus a representative steam engine), fountain, and monuments.
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Templo de La Virgen De La Inmaculada Concepción de San Diego
This church's construction also began in the 1600s by the Franciscans of San Diego. There is a convent of sorts attached.
Nothing too amazing here: just a duck pond and some rabbits/aviary on display.
Home field for the top-tier professional soccer team Impulsora del Deportivo Necaxa (always known simply as Necaxa, except among local fans who have a variety of nicknames, like Los Rayos). The team plays year-round (two seasons annually) in the Liga MX. The stadium has a unique design with an odd upper deck that partially wraps at one end of the field. It was built in 2002 and seats 23,000
Magnificent new contemporary art museum that is itself a work of post-modern art (if you consider architecture an art). The site was a former workshop for repairing locomotives, and there's places where glass floors let you see remnants of the original tracks. The exhibit features mostly rotating exhibits, which seem to change frequently so it's a new experience every time you visit.
An intriguing residence (or, yeah, castle) built for a family that emigrated here from Scotland. It even has a moat, but it probably has lost a bit of luster over the years.
Museo de Aguascalientes
Built out of pink quarry. It has a collection of 20th century arts including the works of Saturnio Herran, who was born in Aguascalientes. Herran's arts include portraits of the everyday citizen painted with a high degree of sensitivity toward his subjects. His work is very similar to art nouveau.