Central Trelew is manageable by taxi or on foot for the museum, plaza, and lagoon. The city is spread out enough that walking everywhere can become dull, especially in wind.
For Bryn Gwyn, Gaiman, Rawson, Playa Union, or Punta Tombo, use a rental car, remise, or tour. Public transport exists between the main valley towns, but it is not built around sightseeing.