Your product or solution and a wide variety of peers and competitors, a big table or blank surface to display the options in an entirely random way, a team member or two to observe, record and ask follow up questions of users after they have sorted the products. If you’re asking to sort apps you could have users sort paper logos whereas if you’re creating beverages then you’ll want to include your product in its expected packaging among a number of beverages.
Over and over from beginning to end. Observing your users as they confront the problem you're trying to solve, whether within the context of your prototype or not, will always yield useful insights and help to validate or invalidate your assumptions.
The goal of both the Lean Startup and Design Thinking processes is to create ventures, products and services that keep the user at the centre of the design and the business. By testing products with users at every stage and without interference or interruption we get as honest an insight as possible in to the experience of the problem and the solution. This reveals the greatest opportunities for meaningful interaction and the areas that ventures can disrupt existing experiences and markets.
Your user, your prototype and your five senses - use as many as possible, quietly, to gain insights into your user and how they are experiencing your problem and solution.