A Flash or Speedlight
For all my travels in the past decade, I have rarely found myself wanting to have a strobe along for the ride, especially due to the added weight involved. Yet I still tend to bring just one flash and I have been happy every time I did.
What I have found valuable when bringing a strobe is to make sure it is one that I can use off-camera. A flash that can be remotely triggered either with a tether or a smaller unit on the camera (either the built-in flash or a unit that attaches to the hotshoe). This has allowed me the greatest flexibility to add well-directed light to my travel photos.
A good travel tripod is a thing of beauty. These units, built for packability and lightweight, have been following me on more and more trips abroad.
My main issue with packing a full-size tripod, even if it is carbon fiber, is the need to take larger luggage. The legs of a super sturdy tripod are often longer, even when broken down, than the luggage I wish to take.
It’s a balancing act and it matters what you intend to shoot. If you have space, bring the monster tripod, especially if you are accustomed to hauling it around with you near home. If you have limited space, but still want stability in your landscape or astrophotography shots, consider something along the lines of the MeFOTO Globetrotter. It packs small, can double as a monopod and has a fully functional ball head.