If you're using a simple passive thermal battery setup and need to increase your thermal mass, the most popular and simplest option is to add large barrels of water, painted black. These are cheap, easy to move in, and as long as they're exposed to plenty of sunlight, they'll do their work. Of course, barrels of water take up valuable greenhouse space, so that may not seem like the ideal solution. Another type of thermal mass that's easy to work with is brick. Deep red bricks absorb far more heat than white cement or other white stones.
In some locations in China, tall brick walls built directly behind the greenhouse itself serve double duty, protecting greenhouses from cold northerly winds during the day, while absorbing plenty of sunlight from the southern exposure. Overnight, the brick radiates the energy back out, which warms the adjacent greenhouse. Alternatively, bricks can be used within the structure itself, for knee walls, flooring, and any other sun-exposed point.
