Other possibilities:
- The Lost Expedition (challenging)
- The Crew: The Quest for Planet Nine
- Geometric Art
- Space Hulk: Death Angel (if they don’t mind the theme and the high difficulty)
- Solar Storm (I’ve never played it, but it sounds a bit like a board-less sci-fi Pandemic in a smaller box)
- The Grizzled (I’ve never played this either)
Mind you, there’s a lot of bigger-box games that can be very compact if you don’t actually take the box. A flat board and a few zip-lock bags of components (ignoring any for 3+ players) can often be packed away very easily in one’s luggage!
Sherlock Holmes: Consulting Detective can be just the rules and whichever episode(s) you choose to bring, for instance.