Portable chargers are great and underrated. For $20 you can get a little box that can charge a phone ~5x or laptop ~1x. [1] You can pay a bit more for one with built-in wall-charging and cables. It's great throwing one in my backpack and knowing I don't have to ration or worry, especially when traveling. And if the kids tablets are running low, a friend needs a charge, or there's not a good place to plug my phone in overnight it's not an issue.
In general 27,000 mAh is a good upper size: past that and your airline might have limits (though the FAA allows airlines to permit up to 44,000 mAh). And once you're getting that big it probably makes more sense to get two for the flexibility and redundancy benefits.
I first got one in 2017, and it's still working well (though the USB-C port has gotten slightly loose):
I've since gotten a couple more so the kids can have separate ones for their tablets on long trips.
[1] The math here is terrible: capacity should be measured in
watt-hours, but instead everyone gives it in milliamp hours relative
to some implied voltage. For USB power banks that's (always?) 3.6V,
so 26,800 mAh is 96 Wh. My phone is 4,385
mAh at 3.85V, so 17 Wh. My laptop is
helpfully spec'd in
Wh: 99.6 Wh. Note that you do lose some capacity to conversion
losses, so discount a bit.