I am trying to figure out how to make a world where flight is a lot easier. How much easier could it be on a human (or near human) habitable world? Lower gravitational force might help although too low would presumably be untenable due to loss of the atmosphere? A higher pressure atmosphere would help but presumably this would be less likely on a smaller world? Heaver gases such as Krypton and Xenon – but are large quantities of these elements realistic? Are there any other possibilities? Finally in order to make the situation more interesting I would like to ensure there are some very high mountains and very deep canyons how high / deep could these reasonably be?