Laptops are smaller and lighter than they've ever been, and even mid-range devices finally have enough power to drive external displays. The problem: monitors are big and heavy. A new breed of portable monitors has started popping up, though. For example: the TAIHE Gemini on Kickstarter. It's not the first portable monitor, but it's among the cheapest and most capable.
The TAIHE is a portable 15.6-inch IPS LCD that comes in two variants. There's a 1080p touchscreen model and a 4K non-touch display. Both also have HDR support. If you're planning to use it on a desktop operating system like Mac OS, the touch functionality might not be worthwhile. Meanwhile, a smartphone or Chromebook would make use of touch.
This monitor looks a bit like the "tablet" portion of a Microsoft Surface. There's a 180-degree kickstand on the back, allowing you to prop it up next to your computer. You can also rotate it to use the monitor vertically if you're going to scroll through long documents. The monitors are 8-10mm thick and weigh 2 pounds, so they'll take up about as much space as another laptop.
Like most portable monitors, the TAIHE relies on a USB Type-C connection. That means you can connect it to a newer MacBook, Chromebook, or Windows laptop. It also works with certain Android phones like those from Samsung and Huawei. It will also work with a Nintendo Switch, but you will want to double-check your cable options -- the Switch is notoriously picky. The USB-C port is the easiest way to use this monitor on the go, but you also have an HDMI or mini HDMI (depending on the version).
Both versions of the TAIHE Gemini have an integrated 5,000mAh battery. That's good enough to power the monitor for five hours, presumably at low brightness (oddly, the designers quote the same battery life for the HD and 4K versions). You can plug the power cable in if there's an available outlet, but your laptop probably won't last much longer than the display without a recharge. You can also use the monitor as a power bank to recharge your phone or tablet.
The super-cheap early bird rewards are all gone, but you can still get the 1080p TAIHE for $219. That's cheaper than similar devices in this category. The 4K version is $299, which is tougher to justify. The campaign runs for several more weeks, but it smashed through the initial $10,000 goal with more than half a million so far. The designers claim a shipping date of May 2019.