...planting strawberries. Growing space is at a premium and you can never have too many strawberries. Plus my LEED certified husband recommended it; plants and soil act as great insulation for the greenhouse.
The strawberries will send out tons of runners that will carpet the entire roof, holding the soil and gravel in place. Want to see pretty pics of roof planting around the world?
p.s. Props and a nod to this lady's mom who planted cantaloupe and corn on their shed's roof and no expert had to tell her if it was a good idea. =)