This mod allows you to sit on slabs, stairs, carpets, and beds. To sit on a block, simply stand close enough to it and right-click on its top or side. Sitting on a bed is a bit different as you need to stand far enough away from it. In the configuration file, any block type can be disabled as a seat.
Furthermore, there is a block tag called kiwi:sittable, which allows you to define any block as a seat, and the seat's height is automatically detected.
