After upgrading my laptop, I spent months feeling bad that I hadn't yet sold the old one. It sat around for months, until one day when a friend was over to work on a writing project. He hadn't brought his computer, so I fired up the spare laptop, whisking away my guilt. That spare computer has now become a dedicated guest computer.
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Many of us have an old "backup" laptop sitting around. The guest computer is just a more useful version of that, logged out of most of your personal accounts so friends can easily access their own. The guest computer is helpful when a friend drops by, but it's even better for when you have guests staying with you.
On short trips, a laptop can weigh down an otherwise light packer. It also adds a mental burden, a concession to work or other commitments. It's an admission that smartphones can't really do everything, even for a weekend. Let your houseguests off the hook: Before they leave home, let them know you have a laptop they can borrow. If they can set up Remote Desktop or log into Dropbox, they will even have access to their files. They don't have to worry about logging you out, the way they would if they borrowed your "real" computer.
If you don't have a spare computer, buy someone else's. A used PC laptop or a used Macbook Air can cost only a few hundred dollars on eBay. For most guests' purposes, any machine that can run web apps will do. It's still much better than squinting into their phones or lugging a computer through airport security. And you'll never have to read out your Wi-Fi password again.