The Museo Paleontológico Egidio Feruglio, usually shortened to MEF, is the one Trelew sight you should not skip. It is a proper research institution as well as a museum, with Patagonian fossils, life-size reconstructions, and a clear explanation of why this dry corner of Argentina matters so much to paleontology.
The displays are accessible without being childish, and the building is central enough to fit into even a short transfer day. If you only came to the area for whales and penguins, MEF is the place that makes you add dinosaurs to the trip.