With the Sit mod, you can use slabs and stairs as chairs. Just right click on the top right corner of the block to sit on it. At the same time, you should not have items in your hands. To get up, press Shift or another button that you use to exit the minecart. If you want to play with this mod in multiplayer, it must be installed on both the client and the server. If you need a Fabric version of the mod, you can download it here.