Sit Anywhere by Qwazor
Java data pack listed for Minecraft 1.21. Download on the author's site.
⬇ Download on CurseForgeSit Anywhere by Qwazor
This is a simple datapack that allows you to turn anything into a chair. Full survival compatible -- no commands required.
To place a seat, place an armour stand named "Seat" on the block you want to sit on.
To remove a seat, place an armour stand named "Clear Seat" next to the seat you want to remove. This will give you back the "Seat" and "Clear Seat" armour stands.
If you are a server operator (or have cheats enabled on singleplayer) type /function seats:load to enable the /trigger seats_help command.
"/trigger seats_help" will print this in chat: 
- Edition: Minecraft Java
- File type: .jar
- Minecraft versions listed: 1.21, 1.21.1
- How to install: Install the matching mod loader (Forge, Fabric or NeoForge) for your Minecraft version. → Download the .jar. → Put it in the .minecraft/mods folder and launch that loader profile.
- Where to get it: Opens on CurseForge — not every file is mirrored on our own servers.
Install steps are the general flow for this file type — see the MC Java Mods guides for the full walkthrough.
Sit Anywhere by Qwazor is a free Minecraft Java datapack. Compatible with Minecraft 1.21, 1.21.1. Downloaded 335 times (via CurseForge). Download it and open it directly in the game.