folding seat

tri-legged stool

• ALPS Mountaineering — $20, weight limit 250 lbs
• GRAND TRUNK Compass 360° — $60, weight limit 350 lbs
• Walkstool is kind of the 'Caddilac' of these. They're pretty expensive but also pretty sturdy. [1] [2]

similar to a tri-legged stool, but not exactly the same

• Travel Chair // Big Slacker Stool — $42, weight limit 275 lbs