Some offline-friendly FPS games.

• Half-Life 1 and 2
• Borderlands 2 and 3
• Dusk
• SEUM: Speedrunners from Hell

First-person puzzle games (other than Portal 1 and 2, which you’ve heard about enough already):

• Once you’re done both Portals, check out community-made mod campaigns!
  • Portal: Revolution is a prequel to Portal 2 that introduces a few new mechanics.
  • Portal Stories: Mel has pretty much zero new mechanics but cranks up the difficulty.
• Viewfinder
• The Witness