Linux

Ex-corporate refurbished laptop from the last 3 or 4 years for about $300 tops is perfect for this.

I have a second-hand Thinkpad T480s that I love, I bought it for 250$ on ebay and replaced its battery because it was fried (+40$). I use it for school and it works flawlessly, around 8h of battery life in a well-configured OpenSUSE Tumbleweed. According to the specs sheet it shouldn't be, but for some reason it is noticeably lighter than a friend of mine's MacBook Air 2021.

What I really love about it is the ThinkDock Ultra (iirc 30$ on ebay), which lets me place the laptop on my table, and by just sliding a piece of plastic, it connects all of my peripherals in a second. I love this laptop so much that I'll use it until it dies so hard that it can't be fixed at all.

It's not the thinnest thing ever, but I find my old ThinkPad X230 very light and easy to use for extended periods on my lap

When you say "couch" my first thought is a recent-ish Celeron or Pentium Silver fanless laptop. Performance akin to a Core 2 Duo but no fan to get blocked sitting on the couch. Like the Latitude 3210(?)

Laptops that appeal to me are often bottom breathers so it's one thing I miss from my old MB Air.

I bought a used Lenovo ThinkPad X240 Laptop i5 | 8GB RAM | 500GB HDD | for 50$ as a couch laptop to run Linux / Python code. I can browse the internet and it’s light.

Any chromebook that supports Coreboot. Absolutely unrepairable and very low storage, but good Linux support and coreboot!

mrchromebox.tech/devices

The pinebooks are pretty inexpensive. I can't speak to quality or usability though

What price bracket are you looking at? The two laptops that I normally use in that situation is a used Thinkpad X1 Carbon I got on eBay, and a HP Dev One that works pretty well for that.

There are plenty of Chromebooks that can wiped & replaced w/ Linux. I run Debian 12 on my HP x360. Check this list: https://chrultrabook.github.io/docs/docs/supported-devices.html

Thinkpad 11e

mine is an asus e210ma with a samsung nvme added, it's great

https://www.ebay.com/itm/155206247310?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=hHEhCOiITeW&sssrc=4429486&ssuid=h5eIb-AvSIq&var=&widget_ver=artemis&media=COPY

I bought a refurbished dall latitude 7490 for like 270$. For the price it's a powerful machine, 16gb ram and i7 processor. Installed fedora on it and I'm in love with it. For the price it puts out the power I need for software development.

Just buy a tablet at that point.

I use a 2013 macbook air for almost this exact use case. Ask friends and family if they have any old laptops lying around.

As others have mentioned, secondhand laptops and surplus business laptops are very affordable and probably better value for the money than a chromebook. My understanding is that drivers for things like fingerprint sensors, SD card readers, or oddball Wi-Fi chipsets can be issues to watch out for. But personally I don't care about the fingerprint sensor and only the Wi-Fi would be a major issue to me.

A couple years ago now I picked up a used Acer Swift with 8th gen intel and a dent in the back lid for something like $200 to use as my "throw in a backpack for travel" laptop, and it has been working great. In retrospect, I would have looked for something with 16GB of RAM or upgradeable RAM (8GB soldered to the motherboard, ugh), but aside from that minor gripe it has been a good experience.

Pi-top or similar?

I used a Pinebook for that

When you say webapps, may I ask what method you prefer for using PWAs on Linux? Do you install them as apps? If so, how?

I picked up a Black Friday Lenovo ChromeBook (Flex 3) for US $160 and use it essentially the same way you describe. You can load up a Debian-based Linux environment within ChromeOS. It's basically my web-capable thin client.

Would a Framework laptop work?

The ultimate couch laptop will be an M1 MacBook Air as it has no fans and a suped up phone chip so it doesn't heat. It also has amazing battery life... But it's still pretty expensive and it cannot be repaired. Otherwise old MacBooks should be pretty good because most of the Intel models used relatively low end chips because their thermal design was so limited

Something with a spacious keyboard would be great.

I bought a used HP Elitebook on eBay for a similar purpose. I can browse and do video calls on a bigger screen when the fancy strikes. Pretty much any used business laptop should work. I think I paid about $300 for mine and I paid extra for particular hardware I thought was neat but you don't have to. Only thing to keep in mind is the battery will likely be pretty worn.

For the usecase you describe, I'd go with a Chromebook, and build ChromeOS from source myself if that aspect felt important.