Depends on how much money you want to spend. One of the rectilinear zooms would be a good general utility lens (10-20mm, 10-24mm, 12-24mm), given that you have normal to telephoto range covered. If you are wandering around close to the balloon take-off (they usually don't allow people in the balloon takeoff area due to safety), you will need a wide angle to get a decent amount of balloon in the frame. The cheap options are the Rokinons (Bower, Samyang, etc), and remember, the wider you go, the greater the depth of field, so for a lot of superwide manual lenses, you do zone focusing and it works out fine.