* A kit bag: This is a literal answer, as you would sit on a bag containing a kit (like a camping or fishing kit).
* A toolbox: Many toolboxes have a seat on top, and they often contain a "kit" of tools.
* A saddle: Horseback riders sit on saddles, and some saddles have compartments for carrying "kits" of essential riding equipment.
Let me know if you'd like to try another riddle!